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DrB
06-30-2011, 04:01 AM
So I had a thought... Probably not a new one, but I haven't yet come across it exactly. I'd appreciate the feedback of the more experienced/observant cast bullet shooters out there. "Papa smurf" had a related thought so I know others are thinking along the same general lines.

You would think since immediately prior to departure from the bore the base edge of a bullet is confining the gas pressure and restraining inertial wobble that a major impact to accuracy would be the beginning of blow out of this trailing edge due to gas pressure (+ other forces), and subsequent net off axis forces imparted to the bullet. Asymmetric venting during departure would impart forces, and further erosion/scarring could additionally influence the creation of net off-axis forces. Non uniform release of inertial forces could also tend to throw the shot.

Trailing edge failure could explain a lot of the correlation between bullet hardness (and gas checking) and better accuracy at higher velocities (and higher muzzle pressure). A stronger bullet trailing edge should result in smaller imperfections at departure, more uniform mechanical release, and thus less net off axis forces, and better accuracy.

It would be neat to examine bases of plain base lead bullets below and well above the velocity where accuracy had significantly degraded, if you could recover them without much impact damage and look for gas erosion/scarring/fracture of the base surface, and particularly imperfections of the base edge where it departs the muzzle.

Any thoughts, observations, references to prior discussion of same?

Anyone ever shot a uniformly soft lead bullet vs. one with just the very base quench hardened? Or how about a gas check that was just a crimped on ring (a cup with no bottom, but a perfectly flat bottom edge) extending just past the base of the lead bullet?

Best regards,
DrB

firefly1957
06-30-2011, 04:22 AM
Have you ever heard of Dr. Mann's the flight of the bullet he used soft lead but altered the bullet base to shoe how it effected the bullets flight. His work was with a 32-40 if I remember correctly.

DrB
06-30-2011, 04:36 AM
Was he the fellow who categorized types of imperfections vs. accuracy impact? Wrinkled, nose defects, base defects, etc? If so, I'm familiar with the experiment generalities but never came across the actual article (I would love to know where to get it). I did read one, once, where a fellow took some soft pointed jacketed spitzers, deliberately mutilated the soft lead tips, and concluded the impact to accuracy was minimal.

As an aside, btw, that result makes sense to me as the asymmetric forces imparted in flight are generally averaged out by bullet rotation (except for variations in total drag), whereas base defects and associated separation forces are lateral kicks that aren't averaged out by rotation of the asymmetric force.

Now at longer ranges, it seems to me you'd expect nose mutilation to result in increased vertical stringing due to velocity variation... How observable would it be at what range? I dunno... Have to run a calc on that sometime. (or profit from the wealth of experience present here) :)

Bret4207
06-30-2011, 06:56 AM
"The Bullets Flight" by Franklin Mann. It's a pretty good sized book, not an article. I've also seen some modern statistical type experiments done where the author said mutilated boolits made no difference, but his data showed something else altogether!

"A stronger bullet trailing edge should result in smaller imperfections at departure, more uniform mechanical release, and thus less net off axis forces, and better accuracy."

Well, given the choice do we want a stronger edge or a more perfect edge? I think the later. My thinking is that by the time the boolit reaches the muzzle the vast majority of the pressure has dropped significantly. Different powders.barrels lengths are going to affect this greatly. I suppose there are nifty instruments available to the scientific community to measure this and some of that higher speed stop action photography would help too.

I mentioned a more perfect edge at the start of the last para. I've found, thanks to the boys here, that boolit bases are often not aligned with the length of the boolit. I've found they can also be seated crooked very easily, even wad cutters, and that alignment is far more important with cast than with jacketed.

I agree with the basic idea you have, that any imperfection of the base will affect the boolit to a degree, more or less. I'm just not sure that relating it to "hardness" without specifying all the other variables involved will sit well.

Bass Ackward
06-30-2011, 07:09 AM
What happens to a bullets base depends more on what happens to the nose and if you are accelerating it when they meet.

The nose has to establish an air flow pattern and break the barrier. This must happen before the bullet exits and you lose control. This force determines how strong the lead has to be. So the sooner you can get airflow established, the more of the slug can remain in the pipe for strength to grip the rifling and navigate this turbulence. If you have a copper check, this can add strength at this critical launch time.

Another issue is barrel length. Are you still accelerating the slug or is it up to speed? Cause with what is happening to the nose when it hits air, (friction) you could begin to strip the base as only some of it remains in the pipe. (acceleration force) Now you are gas cutting.

All of this goes to the old analogies of: shoot the heaviest for caliber bullets possible (longest), use the longest barrels for lead, and shoot fast powders so that acceleration is done before exit and pressure has been dropping to affect the base.

Obviously nose shape (design) plays a role in what you need from a design / hardness level.

It's all in the launch.

44man
06-30-2011, 08:07 AM
I have cans of recovered boolits. I found none with any base damage. Others have shot here with softer lead and I do find skid past the base band and a few that are gas cut up the sides but the base itself will be damaged at the very edges and might result in a poor exit.
My belief is all skid must be stopped at the base band and a little at the front of the boolit is harmless. Enter the gas check.
Bases must be square to the boolit axis.
Before I cut a mold with my cherries, I mill the top surface of the blocks while in the vise and they are never removed until finished. A step I feel is important.
Next is to engage the rifling on center. The reason for a little cylinder play in a revolver and a boolit tough enough to pull the cylinder into alignment. Even fitting a super tight after market pin can take away some accuracy. Too tight all around can cause wear in the throats, cone and rifling entry unless all is in perfect alignment.
Once worn, no boolit can get a straight entry, even those seated with zero run out.
While the base is the most important, there are other factors to consider.
Bass is correct, it is the launch. Anytime you make the boolit conform to the gun instead of making the gun conform to the boolit, you lose.

PacMan
06-30-2011, 09:19 AM
I have very limited experience shooting cast but i have put some thought into this off and on.
One thing that i have found is that no matter how hard the pb bullet is when it exits the bore it's base has changed.Really has no other choice.A bullet sized to grove dia.shows less change than one sized over grove dia. The larger the dia. the more diffrence. Exactly how this affects accuracy i dont know.
Dwight

Gswain
06-30-2011, 09:28 AM
Another thing to consider is the pressure curve in relation to when the bullet leaves the barrel. A faster burning powder will spike sooner in the barrel, providing very little forward momentum by the time the bullet has reached the crown. A slower burning powder will not be consumed as early, and will be imparting more pressure to the base of the bullet when it exits the barrel at the crown. If you are using the "optimum" powder for your gun, the one that does its work throughout the length of the barrel, and the pressure curve drops off just before the bullet exits the crown, a slightly deformed base should not impact accuracy much, as there should be a relatively little push that is asymetrical. A very slow powder would definately impact a poorly shaped bullets trajectory than a fast powder, unless I am mistaken.

PacMan
06-30-2011, 09:46 AM
Here while back there was a post about the width of bottom drive bands on pb bullets. I think it was 44Man that stated he had miss cut a cherry and the bullet came out with a real narrow base but it shot really well. My theory on that is that with the narrow drive band there was less change to the base when fired.
Some feel that a really wide base band helps reduce leading by reducing gas cutting.I think that is bull but cant prove it.If the bullet fits width should have nothing to do with stopping gas cutting.

Doc Highwall
06-30-2011, 11:54 AM
I agree with all of the above but what was not mentioned was expansion ratios.

What I mean by this is take a rifle chambered in 308 Winchester with a 22" barrel and a Encore pistol with a 15" barrel.

Same cartridge and same bullet but the pistol will have greater pressure at the muzzle due less barrel length.

Larger caliber cartridges will drop pressure faster due to bore volume per inch of travel.

A 22-250 Rem. has more case volume then a 30-30 Win. and will suffer more from a short barrel then a 30-30 Win. and hand guns are no different.

Char-Gar
06-30-2011, 12:31 PM
When I was 14, I checked out "The Bullet's Flight" by F.W. Mann, and read it cover to cover several times. I have reread it several times since. It has been reprinted several times. I think it is now online, but don't remember where.

Man was a M.D., who also invented a machine that ground chicken bones into meal. He sold his invention for a large sum of money and spent the rest of his life in the study of internal and external ballistics. One of the very first to do so.

If a serious shooter has not read this book, then he has missed on of the foundations stones of what we do. For the cast bullet shooter, it is of particular significance.

Shome10x
06-30-2011, 12:44 PM
Stupid question....

How does the "pressure" differ between a fast or slow powder? If the velocity is the same, how would this affect flight?

Reason I ask, the same bullet can fly extremely well whether "pushed" by Bullseye or Unique. Help me understand the logic...

Thanks,

Chris in MO

W.R.Buchanan
06-30-2011, 01:23 PM
I read an article in Handloader Mag where Dave Scoville took bullets and deformed the tips in a variety of ways, and even loaded some of them backwards. The accuracy degradation was minimal.

Then he deformed the base of the bullet and things went haywire. A minor file flat on one side of the bases made the groups open up to nearly off the paper.

It is not hard to visualize how a small imbalance when amplified by 200,000 rpms will make the bullet go nuts. But the deformation of the base seems to have a much more dramatic effect than anything done with the nose of the bullet.

In my limited experience in Casting bullets the thing I have noticed is the hardest thing to get completely consistant on is "Complete Base Fill Out" to a sharp edge.

I have cast hundreds of .30 &.44 cal boolits both plain base and gas check designs.

Sometimes the base fills fully and the rear edge is sharp all the way around and sometimes you get a variety of incomplete fills, ranging from partial to complete gentle radius'.

I don't worry about the very small complete and consistant radius around the base as it is at least consistant. However I scrap partial radius's, as I know they won't fly strait due to the deformed base, especially the .30 cal ones. I am more leanient on the .44's as most of them are going thru a pistol and it won't be visible except on paper. Cans will never know the difference.

I think that this particular point is the #1 secret to good cast boolit accuracy. I'm sure this is the reason for Nose Pour Moulds.

I also think that this applies to Gas Check Boolits as well. Covering up a malformed base with a gas check does not remove it's existance.

The degree to which the bullet's base departs from the perfect shape would determine its relative place in the group on paper. What I mean by this, is that a perfect bullet would go into a place in the group. The slightly deformed bullet would go else where, thus opening the group. All other things being equal of course.

My personal opinion on relative accuracy is that most of the significant advances in accuracy in the last 20-30 years, have been as a result of improved bullet manufacturing techniques, and not in firearms manufacture.

John Barsness did an article on bullets with several M70 rifles made from different times thruout the last century. I think there were 4 or 5 ranging from prewar to post 64 to newly manufactured. All were .30-06 cal

All of the guns has similar performance when used with the same loads and the newer the bullets, the better the performance. Even generic level bullets grouped tighter than they had in the past. Surprisingly the best shooter of the group was the M70 made in 1936!

This points directly to the bullet and sumarily dismisses the rifle's vintage from the equation.

Better quality bullets shoot better. Obviously better quality Cast Boolits will shoot better than less than perfect ones. I guess if you are going to shoot in a match then all of your boolits would need to be inspected under magnification and weighed to insure consistancy. With respect to cast bullets, just look at the quality of the moulds we can buy now. My new Mihec mould is some of the best machine work I hhave seen, and I own a machine shop! Obviously it will make better bullets that the old Ideal .38 cal mould I have that makes bullets that are .006 out of round. Mould making has gotten better with the advent of newer machinery and the ability to make better cherries, but also mould design has evolved and gotten better too. Thus we make better Boolits

I think you can certainly get into splitting hairs here, but the thing that rises to the top is you need to taylor your boolits to the style of shooting you are going to do with them.

The best boolits obviously will take the most amount of work to produce, and they should be used wisely. The less than perfect ones can still be flung indiscriminately downrange to satisfy our need to hear the Bang, Boom, Crack or Pow!

Your expectations may vary.

Randy

Doc Highwall
06-30-2011, 01:56 PM
First of all you have a time pressure curve with the powder burning.

Second two powders giving the same velocity can have EXTREAM pressure differences.

Then you have the useful case capacity in grains of water along with the volume of the barrel that the powder expands into.

Say money was no object for some tests to be done on powder burning rates.

Now for this test there is only one cartridge 308 Winchester and only one bullet a 175 grain Sierra Match-king and the gun has a 24" barrel.

Now if you told the engineers to put the powders in a order of burning rate at 40,000 PSI you would have a list from fastest to slowest.

Now ask for a list at 50,000 PSI and the list will change in order of burning rate.

Now ask for a list at 60,000 PSI and the list will again change in order of burning rate.

Pressure affects powder burning rate, the more pressure the faster it burns. How fast the pressure drops because of caliber affects this. This is part of the relationship between case capacity and caliber along with barrel length.

Now add all the different calibers and bullet weights and type of guns with different barrel lengths and everything changes again.

Powders sometimes when they get to a certain pressure SPIKE dramatically for a particular caliber, and you will see in the loading manual where it looks like they stopped short with the powder charge with lower pressures compared with other powders and the same bullet because the powder SPIKES with just a little more powder.

This is some of the things that make you scratch your head when talking internal ballistics and why people here at Cast Boolits will ask a lot of questions before answering your questions.

It is very important to give particulars as to the gun, caliber, bullet, and what it is going to be used for to get the best answer for what you want to accomplish.

Char-Gar
06-30-2011, 02:46 PM
Shome.. Not really all that stupid. The speed of which the powder reaches max pressure is very important in cast bullet shooting, moreso than with condom bullets.

The rapid jolt to be base of the bullet that comes from fast powders can cause the bullet to accordian/deform resulting in accuracy downgrade.

The slow shove to the base of the bullets that comes from slower powder will cause less distortion to the bullet and therefore more pressure can be applied.

So, if we have two similar cast bullet that leave the barrel at 2,000 fps one very well might give better accuracy than the other if a fast powder was used for one and a slow powder for the other.

This is the time pressure curve for dummies like me.

W.R.Buchanan
06-30-2011, 03:58 PM
Doc: That relationship you are talking about was related to me by Dave Scoville as "Pressure/Burn Ratio". He actually spent 45 minutes discussing this and some other things with me at the last SHOT Show.

I was trying to get him to do a complete issue of Handloader with articles from all of the staff writers on the subject of why certain powders work well in some applications but not others, and some powders work well across a wide range of applications when others are very narrow in their usages.

Pressure/Burn Ratio was the answer. followed by a long sigh, followed by 30 minutes of discussion of the phenomenon.

His immediate concerns were that the average reader of Handloader would not understand an article this technical. I dis-agreed and then proceeded to rattle off the hi points of the theory he had just spent 1/2 hour relating to me.

I think he was reconsidering when I pitched the idea to Venturino who thought it was a good idea.

Still waiting for that issue, but suffice to say it will be a stand alone issue of that mag similar to other issues that just seem to have more relavent content than the rest.

I will email him and see if their has been any more interest in the subject.

Randy

Harter66
06-30-2011, 05:42 PM
Well I can't get too technical about it but I can give a 2 rifle,2 cal.,2 boolit,3 powder example of nearly identical results.

I loaded a 32 Rem w/324-170 LEE w/Red Dot in .2gn steps w/each step the groups shrank, dramatically at its peak about 1100fps poof the group went off the paper.I switched to Unique same thing to around 1400 fps poof next step no groups. I then moved to 4350 that work up isn't finished, but my notes say the group is closing and I'm over 1800fps ,which is the start of jacket velocities wt/wt.

I had the same results almost to the fps w/ an 06' and a 200 gn spire point boolit.

Some fool GC'd my chrony, kinda took the fun out of the work ups.

Harter66
06-30-2011, 05:58 PM
I suppose the premise i'm applying is fast powder,fast peak,harder boolit hit,more deformation. My above are all plainbased for the record. My suspicion then, was that the powder was peaking out pressure wise and spiking or outrunning its abilities. I lack the terms here but the pressure was no longer pushing the boolit, but pacing it,the causing a double shock wave at the muzzle. So the boolit was pushing 1 as well as being overtaken by 1 then passing through it again.

Bret4207
06-30-2011, 06:34 PM
Doc: That relationship you are talking about was related to me by Dave Scoville as "Pressure/Burn Ratio". He actually spent 45 minutes discussing this and some other things with me at the last SHOT Show.

I was trying to get him to do a complete issue of Handloader with articles from all of the staff writers on the subject of why certain powders work well in some applications but not others, and some powders work well across a wide range of applications when others are very narrow in their usages.

Pressure/Burn Ratio was the answer. followed by a long sigh, followed by 30 minutes of discussion of the phenomenon.

His immediate concerns were that the average reader of Handloader would not understand an article this technical. I dis-agreed and then proceeded to rattle off the hi points of the theory he had just spent 1/2 hour relating to me.

I think he was reconsidering when I pitched the idea to Venturino who thought it was a good idea.

Still waiting for that issue, but suffice to say it will be a stand alone issue of that mag similar to other issues that just seem to have more relavent content than the rest.

I will email him and see if their has been any more interest in the subject.

Randy

Randy, if you can get Scoville to listen you're in line to be Sec State! His attitude about Handloader readers not being smart enough to understand is why Handloader today is a shadow of it's glory years under Dave Wolfe and Ken Howell, back when guys like Scoville were writing highly technical articles!

Michael Petrov
06-30-2011, 06:59 PM
Everyone who shoots cast lead bullets should read Mann's book, at least once.

http://books.google.com/books?id=QdQqAAAAYAAJ&pg=PA92&dq=F.W.Mann&hl=en&ei=QiTETNmkKcGqnAeHpdjPCQ&sa=X&oi=book_result&ct=result&resnum=8&ved=0CEMQ6AEwBw#v=onepage&q=F.W.Mann&f=false

felix
06-30-2011, 07:37 PM
That would be interesting output from someone with the interest to do the reloading as well. I would very much like to see at least a chapter on which loads in which cartridges that go into a SEE condition, i.e., where loads are consistently smooth before and after a load causing a pressure spike. Doing that would be indicative to me as a competent job. ... felix

What Harter66 described above would be classified as a SEE condition as well. A double shock wave would be a positive identification. I wonder how often that occurs and goes undetected, except for the crazy group at the target amongst others as good groups with the same approximate load before and after. ... felix

waksupi
06-30-2011, 08:25 PM
Everyone who shoots cast lead bullets should read Mann's book, at least once.

http://books.google.com/books?id=QdQqAAAAYAAJ&pg=PA92&dq=F.W.Mann&hl=en&ei=QiTETNmkKcGqnAeHpdjPCQ&sa=X&oi=book_result&ct=result&resnum=8&ved=0CEMQ6AEwBw#v=onepage&q=F.W.Mann&f=false

Thanks for the link. I loaded it on my Kindle.

W.R.Buchanan
06-30-2011, 08:30 PM
Bret: I have talked to this person several times and he is in person exactly like he is in print.

When you look at his point about the technical nature of Pressure/Burn Ratio being a little above the average reader, I can easily see his point.

If you remember when I first came to this site I started a thread called "Knowing versus Understanding". It went on for days until I got into educating people on why it works.

The main point of that thread was to get people to study a post before reacting to what they "think" you said. IE understanding what was being said by the author, as opposed to "going off "when the read a word you didn't understand.

You see here everyday how that went ?

What he was saying is that most people would not put out the effort to actually read the article enough times to grasp the overall concept of the very technical subject.

I have to agree with him on that point. They won't . They read something once a decide they know everything about what was written, or go to sleep.

However that doesn't mean that the topic shouldn't be covered in the mag. It would just only appeal to a smaller segment of the readership that are truly interested in understanding all they know about the subject.

Handloader and Rifle are still the best shooting related mags out there, and I get alot of valuable info from them that I feel confident in using. The reason why is that many of the writers have so much experience that they are completely consistant in their views and statements. Thus they become reliable sources for information.

I call them, "them that knows"

I don't always agree with everything they say but I can tell you that I have used alot of their data successfully and it has worked very well. Sources like this cut down on the work of load development considerably, just like this site does.

Citing the Aug 2007 issue of Handloader, the Brian Pearce article on loading the .45-70 for Marlins is a perfect example. There is more useful info on loading for that caliber in one article than I have been able to find everywhere else combined.

I have read that article 100 times easily. The average person would not do this even if the info was" Instructions on "how to $%&*"

You've been around here long enough (14,542 posts definately says something !) that I know you agree with what I'm saying.

Oh, and I would love to kick Hildegard in a @&& as she left my new office.

Randy

DrB
07-01-2011, 12:01 AM
"The Bullets Flight" by Franklin Mann. It's a pretty good sized book, not an article. I've also seen some modern statistical type experiments done where the author said mutilated boolits made no difference, but his data showed something else altogether!

"A stronger bullet trailing edge should result in smaller imperfections at departure, more uniform mechanical release, and thus less net off axis forces, and better accuracy."

Well, given the choice do we want a stronger edge or a more perfect edge? I think the later. My thinking is that by the time the boolit reaches the muzzle the vast majority of the pressure has dropped significantly. Different powders.barrels lengths are going to affect this greatly. I suppose there are nifty instruments available to the scientific community to measure this and some of that higher speed stop action photography would help too.

I mentioned a more perfect edge at the start of the last para. I've found, thanks to the boys here, that boolit bases are often not aligned with the length of the boolit. I've found they can also be seated crooked very easily, even wad cutters, and that alignment is far more important with cast than with jacketed.

I agree with the basic idea you have, that any imperfection of the base will affect the boolit to a degree, more or less. I'm just not sure that relating it to "hardness" without specifying all the other variables involved will sit well.

Thank you for the title/author... I'll add it to my library. :)


Re "do we want a stronger or more perfect edge", I'm suggesting the answer is YES (both, but substitute "sufficiently strong" for "stronger"). If there is correlation between consistent accuracy at increasing velocities and bullet hardness (or checking), and it is for the reason I am proposing, then you want both a more perfect edge and sufficient hardness to maintain it at bullet/bore departure.

So yes, we want the most perfect edge we can get. But if we start with a perfect edge and too soft a base edge metal at sufficiently high bullet base pressure at crown departure, then forces from angular inertia and base pressure may tend to bugger up the base edge and spoil a symmetric departure of the bullet... I suspect the largest factor in most situations would be due to asymmetric gas venting.


This is just a theory I'm proposing here. ANY good theory should be falsifiable by a reasonable experiment. That's why I just flat out loved Molly's thread, here http://castboolits.gunloads.com/showthread.php?t=116741 (:goodpost:)

In like spirit, I suggested a couple of experiments in the OP. Here are some more that could support or discount what I've proposed.

I think you can demonstrate when and to what extent gas jetting at departure can destroy accuracy by baselining an accurate barrel/lead bullet combo for accuracy and then change only the crown at the bore. This would be done by introducing a single small nick at the bore-crown edge that will allow early/asymmetric gas venting during bullet/crown departure.

another experiment:
I'll bet if you create (on an otherwise well performing plain base bullet) a base edge imperfection that allows gas jetting at the crown prior to/at departure (but doesn't affect obturaton of the bore during internal bore travel), you will see significant degradation of accuracy. Also, that this degradation in accuracy will be greater than that seen from creating an imperfection at say the leading edge of the rearmost lube groove that would comparably imbalance the bullet (so if this held true, it would tend to suggest the effect isn't due as much to balance as to departure/venting).

another experiment:
Base edge imperfections should have less impact to inaccuracy when bullet base pressure is lower at bullet/crown departure. What if you fired the same bullet with a created base edge imperfection from the same barrel first unported and heavily ported (and fully deburred)? (oh! for the shop resources and know-how of a P.O. Ackley :)).

HARRYMPOPE
07-01-2011, 12:18 AM
Merrill Martin did alot of experiments shooting bullets into his nail free sawdust box back in the 1980's to study stuff like this.He did stuff as weird as Dr Mann.You can find the articles in Precision Shooting and old Fouling Shots.The Mann book is dry and i believe more of a historical read(akin to Macbeth) these days.Still i have read it many times.I think the EH Harrison series in the American Rifleman on cast bullets one of the better ones for modern times.

HMP

DrB
07-01-2011, 12:47 AM
What happens to a bullets base depends more on what happens to the nose and if you are accelerating it when they meet.

The nose has to establish an air flow pattern and break the barrier. This must happen before the bullet exits and you lose control. This force determines how strong the lead has to be. So the sooner you can get airflow established, the more of the slug can remain in the pipe for strength to grip the rifling and navigate this turbulence. If you have a copper check, this can add strength at this critical launch time.

Another issue is barrel length. Are you still accelerating the slug or is it up to speed? Cause with what is happening to the nose when it hits air, (friction) you could begin to strip the base as only some of it remains in the pipe. (acceleration force) Now you are gas cutting.

All of this goes to the old analogies of: shoot the heaviest for caliber bullets possible (longest), use the longest barrels for lead, and shoot fast powders so that acceleration is done before exit and pressure has been dropping to affect the base.

Obviously nose shape (design) plays a role in what you need from a design / hardness level.

It's all in the launch.

BA, I've had years and years of fluid dynamics (so I would hope that I would have the capacity to understand what you are saying eventually if we iterate)... But I really don't understand what you are saying above you think is happening?

Stagnation pressure on the nose of the bullet at say 1800 fps would be something less than 64psi from freestream flow, and generally much lower (prior to muzzle blast... and I would suspect probably less than 64 psi due to the surrounding gases at the nose of the bullet already moving with/past the bullet -- that is, it isn't really freestream flow at the nose even when the nose has poked out past the crown).

On the base of the bullet I would expect anything from at least one to two orders of magnitude more pressure (10x60psi to 100x60psi or more) from the combustion gases, depending. It would be interesting to check this with an internal ballistic computer as I am just throwing that out as a swag, but my point is that I really think venting at departure, muzzle whip timing of departure via bullet velocity, and post departure flight with an upset or unbalanced bullet have more to do with accuracy than variations in the free air flowfield around the bullet nose just prior to departure. The magnitude of the forces just shouldn't be large enough, I think, to be of comparable significance?

Maybe I could understand what you are getting at better if you related the sort of things you've physically seen that you think are explained by your post?

Best regards, DrB

DrB
07-01-2011, 12:55 AM
Merrill Martin did alot of experiments shooting bullets into his nail free sawdust box back in the 1980's to study stuff like this.He did stuff as weird as Dr Mann.You can find the articles in Precision Shooting and old Fouling Shots.The Mann book is dry and i believe more of a historical read(akin to Macbeth) these days.Still i have read it many times.I think the EH Harrison series in the American Rifleman on cast bullets one of the better ones for modern times.

HMP

Is there a way to get a compilation (other than by hand scanning at the library) or at least a list of his articles by date? Is the american rifleman available as a digital archives?

DrB
07-01-2011, 01:26 AM
I suppose the premise i'm applying is fast powder,fast peak,harder boolit hit,more deformation. My above are all plainbased for the record. My suspicion then, was that the powder was peaking out pressure wise and spiking or outrunning its abilities. I lack the terms here but the pressure was no longer pushing the boolit, but pacing it,the causing a double shock wave at the muzzle. So the boolit was pushing 1 as well as being overtaken by 1 then passing through it again.

Interesting results! Thanks!!!

So regarding the double shock, thing. Compression waves turn into (condense into) shock waves when we're talking about significant changes in pressure communicated by the wave. This is because when you compress a gas you heat it, heating it raises the speed of sound, and any following compression waves are going to tend to catch up with the leading waves until they condense into a shock.

So when the combusting/expanding gases are communicating pressure up the bore to the base of the bullet, it is happening as shock waves all the time. They are ubiquitous. Expansion waves are likewise being communicated from the base of the moving bullet back down the barrel to the chamber (there's no such thing as an expansion shock because an expansion wave lowers the speed of sound).

Now, about what you observed with your loads. That sounds consistent to me with the idea that accuracy tends to be destroyed when the stress imparted to the bullet metal (by the rifling trying to impart angular acceleration/spin) exceeds the stress the bullet metal can withstand. The engraved lands start opening up (taking up more and more of the bullets circumference instead of just the physical rifling land width).

Faster burning powders can kick a bullet so hard initially that the rifling skids, you get poor obturation and maybe some of the asymmetric venting departure effects I've been suggesting. A slower burn rate may let you push to higher velocities by not over torquing the engraved lands on the bullet. A slower yet powder may let you get faster still. I don't see how your observations necessarily are contrary to what I was proposing in the OP.

Best regards,
DrB

303Guy
07-01-2011, 06:10 AM
Here is flame cutting that occured quite far up the boolit shank.

http://i388.photobucket.com/albums/oo327/303Guy/MVC-522F.jpg

All the leaking gasses got channeled into one jet by the groove in the boolit maybe?

I've discovered that a light load very fast powder can expand a case neck more than a stiffer charge of slow powder. Primer flattening supports the supposition that pressures were very mild indeed. How do I know the necks were expanding more? Rust damage in the neck area of the neck was gripping those case necks which doesn't happen with more powerful and higher pressure slow powder loads.

Bret4207
07-01-2011, 06:30 AM
Bret: I have talked to this person several times and he is in person exactly like he is in print.




Arrogant and egotistical?

It make take the average HL reader a couple times going over something, but I think raising the bar rather than lowering it is a worthy idea.

Bret4207
07-01-2011, 06:44 AM
Thank you for the title/author... I'll add it to my library. :)


Re "do we want a stronger or more perfect edge", I'm suggesting the answer is YES (both, but substitute "sufficiently strong" for "stronger"). If there is correlation between consistent accuracy at increasing velocities and bullet hardness (or checking), and it is for the reason I am proposing, then you want both a more perfect edge and sufficient hardness to maintain it at bullet/bore departure.

So yes, we want the most perfect edge we can get. But if we start with a perfect edge and too soft a base edge metal at sufficiently high bullet base pressure at crown departure, then forces from angular inertia and base pressure may tend to bugger up the base edge and spoil a symmetric departure of the bullet... I suspect the largest factor in most situations would be due to asymmetric gas venting.


This is just a theory I'm proposing here. ANY good theory should be falsifiable by a reasonable experiment. That's why I just flat out loved Molly's thread, here http://castboolits.gunloads.com/showthread.php?t=116741 (:goodpost:)

In like spirit, I suggested a couple of experiments in the OP. Here are some more that could support or discount what I've proposed.

I think you can demonstrate when and to what extent gas jetting at departure can destroy accuracy by baselining an accurate barrel/lead bullet combo for accuracy and then change only the crown at the bore. This would be done by introducing a single small nick at the bore-crown edge that will allow early/asymmetric gas venting during bullet/crown departure.

another experiment:
I'll bet if you create (on an otherwise well performing plain base bullet) a base edge imperfection that allows gas jetting at the crown prior to/at departure (but doesn't affect obturaton of the bore during internal bore travel), you will see significant degradation of accuracy. Also, that this degradation in accuracy will be greater than that seen from creating an imperfection at say the leading edge of the rearmost lube groove that would comparably imbalance the bullet (so if this held true, it would tend to suggest the effect isn't due as much to balance as to departure/venting).

another experiment:
Base edge imperfections should have less impact to inaccuracy when bullet base pressure is lower at bullet/crown departure. What if you fired the same bullet with a created base edge imperfection from the same barrel first unported and heavily ported (and fully deburred)? (oh! for the shop resources and know-how of a P.O. Ackley :)).

I agree. but I think there's more to it than JUST another "harder is better" theory. Crown condition, rifling depth and condition, rifling form and variations, pressure curve, base style and diameter and base diameter vs groove depth are a few of the variable I can think of. And then there's alloy composition and barrel condition too. There are more I'm sure.

I'm not fighting your idea, I just think you need to include the other factors too. If it's just "harder is better"... well, they make jacketed bullets for that.

Bass Ackward
07-01-2011, 07:01 AM
Maybe I could understand what you are getting at better if you related the sort of things you've physically seen that you think are explained by your post?


The real problem with understanding is the magnitude of influences that occur in trying to replicate exactness too understand.

1. We know that going up through the barrier is destabilizing and coming back through it as well.
I got that from a 50s TV show on Joe Walker about design problems. (neighbor) And the barrier is broken at the muzzle. You will hear the fracture as you run ladder testing. You go from poom to crack. Not only do you have force, but friction and heat.

2. Accuracy zones (with velocity) goes up as bore diameter declines regardless of twist rate. 45 caliber is 1300 fps and 22 caliber is where you want it really. Strange thing is that pressure drops (muzzle pressure the least) most in 45 caliber and least in 22. Think about that one as it covers your proposed testing.

3. If you bring pressure up slowly enough with a PB so as not to ruin a base, then pure lead will fail at one velocity and you can get more velocity as you harden. The narrowest window again is 45 caliber where you have very little advantage to over harden and the widest 22 where hardening pays off big time.

4. In the same caliber, accuracy points decline (in velocity) as twist rate increases.

5. I ran penetration testing in gelatin awhile back using pure lead on a 150 gr spitzer design to cut back on the amount of gelatin needed. What I found was that penetration dropped and became erratic as velocity increased beyond the bullets ability to hold during launch. Bullets did not mushroom from the spitzer design really, but I concluded that as velocity increased RPMS were actually dropping, thus the loss of stabilization. The gas checks appeared to hold the rifling, but rotated on the base. (no longer aligned with the bullet) If you picture that in your mind, there is only one way it can happen really.

That's from too many years of experience.

felix
07-01-2011, 11:42 AM
We can never assume the effective boolit "twist" at launch is ever equal to the "barrel" twist, except maybe when the target consistently displays consistent groups. Even then, it would be nice to know the boolit's actual rotation just for the fun of it (proof?). ... felix

W.R.Buchanan
07-01-2011, 01:27 PM
bret: I agree that raising the bar should be the intended goal of any magazine, and they all profess to be educating the masses in their own ways.

I personally think that the articles I requested would appeal to a larger number than what Scoville thought, and if explained in a manner that was understandable to more people, (IE using discriptive terms that are more easily understood than the strictly technical jargon that the "scientifics" use) would certainly be useful in raising the bar you spoke of.

I understand the point about arrogant and egotistical, but I personally feel that those terms only apply to those who can't back up what they say with some significant level of expertise.

Regardless of what you think of him, Muhamad Ali/Cassius Clay was arguably the greatest Boxer of all time. Reason being he pretty much did what he said he was going to do.

Now if you really want to talk arrogant and egotistical you need look no further than our current president. He is the very personification of the words. :takinWiz:

I don't think Scoville rises to that level, although in his case, speaking from his 50 years of experience, anyone could become a bit jaded when answering the same questions for the thousandth time. I would classify him more as "gruff and firm in his convictions". [smilie=p:

I can see how you might not like his tone, but you can't argue with his qualifications.

Randy

felix
07-01-2011, 01:51 PM
Yes, Randy, there is no real argument in what you have said, but I will say trying to explain something to anyone without having the experience to comprehend/understand is useless. For this reason I agree with Scoville. If he should do such an article, it would be best for it to be published somewhere else, like in Precision Shooting, or in a professional periodical dealing with thermodynamics or something similar. ... felix

W.R.Buchanan
07-01-2011, 05:47 PM
Felix: There was an article published in a more recent Handloader by a guy who was attempting to quantify the efficiency of different powders with relation to different cartridges. All .30 caliber cartridges of one style or another ranging from .30x1.5 to.30-378

At least that's what I think he was trying to do?

The article was 5 pages long and contained a bunch of graphs.

I have no idea what the results of this "experiment" was as the last paragraph makes very little sense.

I have tried to read this article several times only to get bogged down by nebulous terms the meaning of which, can not be extrapolated from the context. And I doubt they were used correctly anyway.

In fact he did state the results of this test. "Velocity is a function of case capacity" "Some cartridges are more efficient than others" My reaction was NO S*&%?

Point is for such a technical article a Glossary of terms should have been included and the editor should have simplified the content to where the actual results would have been a little easier to deduce.

Taking such lengths to state an obvious fact tells me this guy really doesn't understand all he knows about this subject. In the end he couldn't apply an "efficiency number?" to each cartridge so you could tell what the relative efficiency betweeen cartridges was. So what was the point?

"Some were more efficient than others" really didn't cut it.

Even with my limited knowledge of the nuts and bolts of Handloading I was able to deduce that I would not be using any data published by this guy. I don't believe he knows what he is talking about.

If you actually know what you are talking about (any subject whatsoever) you should be able to transfer that knowledge in clear concise terms that your audience will understand. This was NOT accomplished in this article.

Brian Pearce, Mike Venturino, John Barsness, Dave Scoville, John Haviland, Charles Petty and others who contribute to those mags, can do this.

As a result I consider them to be "them that knows"

Charletons typically try to use at least one undfinable term in every sentence. This insures that unless you know the meanings of the words they are using wrongly, or know the subject better than they do, that you can't tripp them up. (read that as , bedazzled by BS) most people won't say anything. But some will and this is how I have been able to avoid the Liberal Mind-rape of living in CA for 55 years.

Maybe you're right and that type of article would be better served in a more technically oriented source. My problem is,,, I thought Handloader was that source.

Randy

Bret4207
07-01-2011, 07:03 PM
Randy, I can accept gruff and jaded. Scoville used to write some really good articles as did guys like Jim Carmichael, Rick Jamison, of course Ken Waters, Mike V, and Al Miller. Somewhere along the line HL simply got dumbed down a notch. Having most of the issues from #1 on you can see the progression. It saddens me when the current editor has no interest in putting the mag back up to it's former level.

Guys like Felix (an actual, honest to God rocket scientist!) and Dr B with their scientific background can often interpret for us lesser beings what the Mr. Spocks of the world are trying to say. Handy to have guys like that around!

DrB
07-01-2011, 11:08 PM
I agree. but I think there's more to it than JUST another "harder is better" theory. Crown condition, rifling depth and condition, rifling form and variations, pressure curve, base style and diameter and base diameter vs groove depth are a few of the variable I can think of. And then there's alloy composition and barrel condition too. There are more I'm sure.

I'm not fighting your idea, I just think you need to include the other factors too. If it's just "harder is better"... well, they make jacketed bullets for that.

Bret, I didn't think you were fighting the idea. I do not believe anything about ballistics is "simple" except when a large number of things are already pretty close to perfect. :)

I'm not trying to propose a grand unified theory of cast bullet accuracy. :) As I mentioned, I loved Mollys thread, and not only can the theories she's proposed appeal to me, I can go out and try to test them for myself as she has proposed.

I've been doing a lot of reading on cast boolits and elsewhere lately, and was wondering about the correlation between increasing velocity and increasing hardness (or checking) with accurate loads. The analog of an imperfect crown with an imperfect bullet base got me wondering as to how much base edge quality at departure might explain.

And no one has to worry about getting me riled. :) Reality is. I hope someone (or us collectively) has data they are willing to share that would support or discount this particular notion. There is always something new to learn (or unlearn).

DrB
07-01-2011, 11:38 PM
We can never assume the effective boolit "twist" at launch is ever equal to the "barrel" twist, except maybe when the target consistently displays consistent groups. Even then, it would be nice to know the boolit's actual rotation just for the fun of it (proof?). ... felix

Can't you have pretty stout proof in the form of a recovered bullet with engraving marks intact? As long as there isn't evidence of skidding/stripping, you should be able to calculate nominal spin rate.

Engraved groove depth as compared to rifling width should even allow you to calculate a maximum error (minimum spin rate) with a few assumptions? Haven't quite thought this through enough to propose a formula when there is widening of the lands evident.

Nominal spin rate in rotations per minute = (velocity ft/sec)/(twist inch/turn)*(12 inch/ft)*(60 sec/min)

For experimental measurement you could do high speed photography with a marked bullet, measure total reflected light levels from a scene with a marked bullet vs. time, or you might be able to do it acoustically with a symmetrically spooned nose or such. Probably not necessarily cheap but very doable if you had a budget.

felix
07-02-2011, 12:13 AM
Time + Money + an obnoxious attitude for an objective = accurate results. Most folks give up too soon for one reason or another, either good or bad. My experience has been not obtaining external ideas to get over an unexpected hurdle. The real excuse for many of these projects has been no profit potential for the results.

You are correct in using a "photo" apparatus for measuring the rotation. The boolit would have to be painted with something "hot" that can be read with the existing particle measuring devices. Finding an appropriately short half-life paint would be prohibitive???? Who wants a forever contaminated barrel? ... felix

DrB
07-02-2011, 01:12 AM
The real problem with understanding is the magnitude of influences that occur in trying to replicate exactness too understand.

1. We know that going up through the barrier is destabilizing and coming back through it as well.
I got that from a 50s TV show on Joe Walker about design problems. (neighbor) And the barrier is broken at the muzzle. You will hear the fracture as you run ladder testing. You go from poom to crack. Not only do you have force, but friction and heat.

2. Accuracy zones (with velocity) goes up as bore diameter declines regardless of twist rate. 45 caliber is 1300 fps and 22 caliber is where you want it really. Strange thing is that pressure drops (muzzle pressure the least) most in 45 caliber and least in 22. Think about that one as it covers your proposed testing.

3. If you bring pressure up slowly enough with a PB so as not to ruin a base, then pure lead will fail at one velocity and you can get more velocity as you harden. The narrowest window again is 45 caliber where you have very little advantage to over harden and the widest 22 where hardening pays off big time.

4. In the same caliber, accuracy points decline (in velocity) as twist rate increases.

5. I ran penetration testing in gelatin awhile back using pure lead on a 150 gr spitzer design to cut back on the amount of gelatin needed. What I found was that penetration dropped and became erratic as velocity increased beyond the bullets ability to hold during launch. Bullets did not mushroom from the spitzer design really, but I concluded that as velocity increased RPMS were actually dropping, thus the loss of stabilization. The gas checks appeared to hold the rifling, but rotated on the base. (no longer aligned with the bullet) If you picture that in your mind, there is only one way it can happen really.

That's from too many years of experience.

1) Ok, so I think when you say barrier you are referring to the sound "barrier?" /EDIT: I think I missed saying something pretty important for the laymen here, about the "sound barrier." The "barrier" is in fact not a barrier at all. Yes, you get an increase in the drag coefficient for a body around Mach 1, but it's not really that much of a "barrier," especially when you're talking about ballistics. Although everyone made a big deal about Yeager breaking the sound barrier, it's not like the engineering community just didn't realize that bullets and other objects travelled faster than the speed of sound (as most everyone here no doubt realizes there are plenty of supersonic blackpowder rifle loads). It was just a little more challenging to push a lightly built self-propelled aircraft faster than Mach 1 than it is a solid bullet shot from a barrel. This was in part because of compressibility impacts on the controllability of the aircraft with the then state of the art, and also with the principle mode of propulsion at the time (the propeller). Props rapidly loose performance and tend to get vibration problems as portions of the blade become supersonic (which inevitably happens as the blade rpm increases and/or aircraft speed increases). /EDIT

Bullets are not very stable regardless of the sound barrier. The effect of supersonic flight is to make them less aerodynamically stable as the center of pressure moves forward (and you want the center of pressure behind the center of gravity for aerodynamic static stability).

We spin long bullets really fast for two principal reasons... To keep the launch end forward despite bad aerodynamic stability, and to average out radially asymmetric aerodynamic forces from bullet surface geometry imperfections.

I think my point regarding the relative magnitude of forces due to ambient gas pressure vs. base pressure still stands. The 64 psi number I stated before was a maximum bound for pressure at the stagnation point on the nose for standard conditions in freeflight. I calculated the "total pressure" as an upper bound, but really it is too high as the total pressure assumes the freestream is isentropically brought to rest, and the gas at the nose of the bullet has experienced shock(s). I'm not even sure though that the nose of a "supersonic" bullet is really moving faster than the freestream until it flies through the first few calibers/gets through some muzzle blast after leaving the bore. You see, the bullet has been pushing air ahead of it down the bore and there is also probably some gas leakage, and so the mouth of the bore is "blowing" prior to the bullets exit (I'm not sure how fast, typically, but it could theoretically be a supersonic jet of gas right before bullet exit /EDIT: Well, actually without leakage gas velocity would have to be as fast as bullet velocity right around exit... with leakage gas velocity blowing out the bore would be higher than bullet velocity immediately prior to bullet exit /EDIT). Then when the main muzzle blast occurs with the uncorking of the bore, the gas around the bullet is traveling faster than it is as the bullet is overtaken by the expanding gases!

ANYWAY, I'm really pretty confident that with a reasonably sealed bore the freestream forces at bullet nose exit (before the tail of the bullet is released by the bore) are negligible compared to those induced by the escaping gas in the muzzle blast when the bore is uncorked. The freeflight/ambient pressures just shouldn't be of the same order of magnitude as those resulting from the pressures in the barrel.

Sorry for going on... Did I correctly understand what you meant by "barrier"?

2) by accuracy zones do you mean the useful velocity range that provides accuracy in a given caliber (and given tailoring of powder to cartridge)?

3) Do you propose this is true for similar designs of cartridges/firearms/bullets? That is, if I took a 50 bmg rifle (instead of a 45 long colt) and shot it with a similar design bullet do you think I'd have significantly different results in terms of improving accuracy with hardness vs velocity than I would in 22 caliber?

4) I don't understand what you mean... Please rephrase? What is an "accuracy point"?

5) neat! This may or may not be the same thing, but this sort of result is VERY well known in the penetrator impact engineering literature. Do the following: calculate the dynamic pressure of your test medium as 1/2 (medium density)*(velocity)^2, for all your test point velocities, and compare the pressures you get with the yield and ultimate stresses (in the same units) for soft lead as you were using. I'll bet your dynamic pressure crossed over the yield and/or ultimate stress for the bullet metal. What happens is you get increasing penetration with increasing velocity up to a point, and then the fluid flow pressures generated by the impacted medium start destroying the penetrator (bullet) which decreases penetration.

I'm not sure penetration here had anything to do with accuracy or the intactness of the bullet edge? It does sound like you were stripping the rifling, though, which ain't good for shooting groups for a bunch o' different reasons.

Thanks BA for expanding on your original post.

Best regards,
DrB

DrB
07-02-2011, 02:10 AM
Here is flame cutting that occured quite far up the boolit shank.

http://i388.photobucket.com/albums/oo327/303Guy/MVC-522F.jpg

All the leaking gasses got channeled into one jet by the groove in the boolit maybe?

I've discovered that a light load very fast powder can expand a case neck more than a stiffer charge of slow powder. Primer flattening supports the supposition that pressures were very mild indeed. How do I know the necks were expanding more? Rust damage in the neck area of the neck was gripping those case necks which doesn't happen with more powerful and higher pressure slow powder loads.

Holy smokes! :) That's cool. Was that a paper patch (i don't see much engraving on the base)?

Seems to me that somehow that groove was pressurized by hot gases (an undersize diameter aft of the lube groove? Or if paper patched a tear or chunk out of the patch?). When the hot gases started leaking through some initially smaller channel they started eroding it more and more. But maybe that's making too much soup from the meat here. :) can't entirely tell texture and depth from the photo, and where gas damage ends and impact damage begins.

What was it shot into, and what kind of load was it?

Regarding neck expansion, wouldn't you expect to see neck expansion before primer sign? If you also weren't seeing primer sign on the slower powder load, then that sounds to me like the light fast powder load reached a higher peak chamber pressure (with less total gas volume generated). The heavier slower powder load may have generated more velocity with a higher generated gas volume, but (because of the slower burn) with a lower peak pressure in the chamber?

Bass Ackward
07-02-2011, 06:43 AM
1) I think my point regarding the relative magnitude of forces due to ambient gas pressure vs. base pressure still stands. The 64 psi number I stated before was a maximum bound for pressure at the stagnation point on the nose for standard conditions in freeflight. I calculated the "total pressure" as an upper bound, but really it is too high as the total pressure assumes the freestream is isentropically brought to rest, and the gas at the nose of the bullet has experienced shock(s). I'm not even sure though that the nose of a "supersonic" bullet is really moving faster than the freestream until it flies through the first few calibers/gets through some muzzle blast after leaving the bore. You see, the bullet has been pushing air ahead of it down the bore and there is also probably some gas leakage, and so the mouth of the bore is "blowing" prior to the bullets exit (I'm not sure how fast, typically, but it could theoretically be a supersonic jet of gas right before bullet exit). Then when the main muzzle blast occurs with the uncorking of the bore, the gas around the bullet is traveling faster than it is as the bullet is overtaken by the expanding gases!

ANYWAY, I'm really pretty confident that with a reasonably sealed bore the freestream forces at bullet nose exit (before the tail of the bullet is released by the bore) are negligible compared to those induced by the escaping gas in the muzzle blast when the bore is uncorked. The freeflight/ambient pressures just shouldn't be of the same order of magnitude as those resulting from the pressures in the barrel.

Sorry for going on... Did I correctly understand what you meant by "barrier"?

2) by accuracy zones do you mean the useful velocity range that provides accuracy in a given caliber (and given tailoring of powder to cartridge)?

3) Do you propose this is true for similar designs of cartridges/firearms/bullets? That is, if I took a 50 bmg rifle (instead of a 45 long colt) and shot it with a similar design bullet do you think I'd have significantly different results in terms of improving accuracy with hardness vs velocity than I would in 22 caliber?

4) I don't understand what you mean... Please rephrase? What is an "accuracy point"?

5) neat! This may or may not be the same thing, but this sort of result is VERY well known in the pernetrator impact engineering literature. Do the following: calculate the dynamic pressure of your test medium as 1/2 (medium density)*(velocity)^2, for all your test point velocities, and compare the pressures you get with the yield and ultimate stresses (in the same units) for soft lead as you were using. I'll bet your dynamic pressure crossed over the yield and/or ultimate stress for the bullet metal. What happens is you get increasing penetration with increasing velocity up to a point, and then the fluid flow pressures generated by the impacted medium start destroying the penetrator (bullet) which decreases penetration.

I'm not sure penetration here had anything to do with accuracy or the intactness of the bullet edge? It does sound like you were stripping the rifling, though, which ain't good for shooting groups for a bunch o' different reasons.

Thanks BA for expanding on your original post.

Best regards,
DrB

1. Yes the sound barrier. We rotate for stabilization. That is to stabilize in air and through other strike materials as well. As I understand center of pressure, it constantly changes in an accelerating and then decelerating object so it passes the CofB twice. Control of it then is irrational and more or less why bullets find comfort zones. But with increased velocity, we constantly encounter increased headwind and the bullets shape and diameter increases these forces enough that we should have taller rifling at the launch.

2. Yes 45 caliber is 1100 to 1300 for easiest accuracy, increases to 1600-1800 for 30 cal, etc. The governing factor is control of the launch. Assuming .004 tall rifling as an industry standard, that represents the lowest percentage of bullet diameter in 45 caliber and the most in 22. Control of the launch. Not muzzle pressure at the release. Form a ratio for .004 / 22 caliber and work that out for 45 caliber and the control launch should be comparable. Still the 45 is going to catch more wind as it may in fact need to be taller even though muzzle pressure will be less. (base shape minimized especially if bullet length can aid transition)(most don't want heavy bullets in 45 caliber)

3. Lost me. What I was getting at is that you can do 1100-1300 in 45 caliber with pure lead or WDWW. With a 22 you can get accuracy from about 800 fps with pure up to 3000 with harder lead. Much wider window. Again because of the rifling height relationship to bore diameter. Not the base edge quality.

4. Relates to the explanation in 2.

5. The moral of that story was supposed to be that many people don't think about metal failure. It doesn't just occur at some point of the bore. It starts at the muzzle and the very back or base of the bullet upon exit. As it does you create a new base edge to release gas regardless of perfect the last one was. And that slug will react to the gas causing wobble if you don't have a wad or something behind it to contain it. In other words, the name gas check is totally deceptive and confusing. It should be thought of and called a launch check.

Bret4207
07-02-2011, 07:57 AM
Bret, I didn't think you were fighting the idea. I do not believe anything about ballistics is "simple" except when a large number of things are already pretty close to perfect. :)

I'm not trying to propose a grand unified theory of cast bullet accuracy. :) As I mentioned, I loved Mollys thread, and not only can the theories she's proposed appeal to me, I can go out and try to test them for myself as she has proposed.

I've been doing a lot of reading on cast boolits and elsewhere lately, and was wondering about the correlation between increasing velocity and increasing hardness (or checking) with accurate loads. The analog of an imperfect crown with an imperfect bullet base got me wondering as to how much base edge quality at departure might explain.

And no one has to worry about getting me riled. :) Reality is. I hope someone (or us collectively) has data they are willing to share that would support or discount this particular notion. There is always something new to learn (or unlearn).


I have no doubt that an absolutely square, perfect base with a perfectly aligned boolit will outshoot a mediocre example given the other variables are consistent. If I follow your thinking, and I have zero background for this, you're saying it would be better to have a base more resistant to abrasion or gas cutting (harder, stronger, tougher, whatever) as the boolit exits the muzzle. This sort of takes me back to the questions people ask about lead alloys "melting", that is- is there time enough for the damage to occur? At even low speeds of say 1300 fps the boolit is only there for a fraction of a fraction of a second. How much time is needed to make a significant amount of damage occur? I have no clue.

I do think the little variables add up in this game. I like fat boolits and deep rifling, but common sense tells me the chances for a bunch of little "tags" ( a shepherds term) are more likely to be hanging off the base of a fat boolit where the displaced metal is dragged backwards. I have to wonder what the effect of a whole mess of those little guys would be.

Keep thinking, this is interesting.

44man
07-02-2011, 11:08 AM
I don't know Bret. I just examined a bunch of both PB and GC boolit bases and they look the same. No "chads" hanging, just a little depression in the GC center and some ripples from rifling. The PB actually still have flat bases and show less ripple. There is a darkness to the lead from the powder but even the little sprue lump is still there.
I can't see any boolit base being damaged at the muzzle. A bad crown can tip a boolit from uneven gas venting but it sure will not melt it or deform it. Not enough heat or pressure left.
But I refuse to shoot dead soft lead and read stories of ported barrels having the ports full of lead. Just maybe the lead is too soft? Silly Putty shoots funny! :veryconfu Maybe Silly Putty will deform at the muzzle.

PacMan
07-02-2011, 01:40 PM
Those "tags" that Bret is talking about is what i was refering to in my post on driving band width.The wider the bottom band on a pb bullet the more it should increase the "tag" size as would a fatter bullet in theory anyway.

There was another post where someone was talking about increased accuarcy with bullets having multiple lube groves whether used or not. I theorize that the reason was the groves give a place for the excess lead to flow but i dont know.

44man
07-02-2011, 02:40 PM
Those little extrusions at the base mean nothing if they are even all around.
The problem at the muzzle will still start at the "LAUNCH" as Bass loves to say. He is correct.
Start a boolit out of line with the bore and it is not aligned at the muzzle.

DrB
07-03-2011, 12:09 AM
Time + Money + an obnoxious attitude for an objective = accurate results. Most folks give up too soon for one reason or another, either good or bad. My experience has been not obtaining external ideas to get over an unexpected hurdle. The real excuse for many of these projects has been no profit potential for the results.

You are correct in using a "photo" apparatus for measuring the rotation. The boolit would have to be painted with something "hot" that can be read with the existing particle measuring devices. Finding an appropriately short half-life paint would be prohibitive???? Who wants a forever contaminated barrel? ... felix

Well, I suppose you might do it that way too, but I don't know why you'd use a radio taggant. I think you might just paint one side of the bullet white and one black, or dull one side for contrast. Sure it'll come off in the high points and places blasted by bore gases, but I think you could get it to adhere to the nose. We paint supersonic airplanes all the time, and the paint just has to stick for a few milliseconds on a bullet (2 revolution*14 inch/revolution/(1800 feet/sec*12 inch/foot)=1 msec.

I'm sure its been done before at the arsenals. I've seen continuous test video of mortar and artillery shells in freeflight (THAT struck me as pretty impressive, but they've been doing that for decades). No reason not to do it with a bullet for a couple of feet (heck of a lot easier, just need a fixed camera and a light source sufficiently bright, like a flashtube, and a "chopper" to pulse the light fast enough to freeze motion).

But anyone can check to see if there was enlarging of grooves on a bullet, and measure speed with a chrony?

PacMan
07-03-2011, 12:41 AM
I agree 44man that start a bullet out off centered,out of ballance,that there is no way for correction. Not knocking Bass but that is not rocket science figuring.Those "tags" or overruns may or may not cause a problem.Like you have said many times you just dont know. There is a good chance you may very well be right in the fact that they do not matter or it could be that we do not really know.I figure those little overruns have been around for a while and are not only expected but accepted as no problem.

Now me I am a firm beliver in not fixing whats not broken.I also know that we do not always know whats not broken.

I would think that until someone designs a pb bullet that will not produce those overuns i not sure that we can be proved either way.
Without a doubt i have been at this a far shorter time than most but since the first bullet that i recovered and saw those little overruns "tags" that sight has stuck in my mind.I have a design in mind that should ellimnate the overun.Thinking of talking to Accurate molds about cutting the mold and give it a try. Only problem is i am not sure what nose profile to use.

Being a simpleton i'm not sure why i am in this discussion anyway.Between Felix and DrB i have been left far behind.

Dwight

DrB
07-03-2011, 12:47 AM
I don't know Bret. I just examined a bunch of both PB and GC boolit bases and they look the same. No "chads" hanging, just a little depression in the GC center and some ripples from rifling. The PB actually still have flat bases and show less ripple. There is a darkness to the lead from the powder but even the little sprue lump is still there.
I can't see any boolit base being damaged at the muzzle. A bad crown can tip a boolit from uneven gas venting but it sure will not melt it or deform it. Not enough heat or pressure left.
But I refuse to shoot dead soft lead and read stories of ported barrels having the ports full of lead. Just maybe the lead is too soft? Silly Putty shoots funny! :veryconfu Maybe Silly Putty will deform at the muzzle.

:)

Hey, can you post before/after pictures and particulars for the load? (load recipe, velocity)
It would also be interesting to see the same with the load pushed to accuracy failure (if that can safely be done with yours).

To my way of thinking, if these bullets you looked at were from a "good" load, you shouldn't see: rifling width enlargement, gas cutting, or changes to the plain base edge. The first two could cause variation in muzzle velocity and therefore changes in muzzle location at departure, and all of them could cause asymmetric venting at departure.

DrB
07-03-2011, 01:53 AM
EDIT/:

So after the original post, below, there seemed to be a fair bit of confusion as to what the heck I was talking about, and what it might mean. My apologies... I'm not trying to write articles for popular consumption here, or for a professional journal, or anything... rather I'm trying to put some thoughts out for discussion amongst whoever may be able and interested enough to parse them, and get some feedback from the community (there's a huge amount of practical experience, here). That said, the intent of the below was bound to be pretty incomprehensible for almost everyone here without some kind of a lead-in. I'm not trying to write posts that are incomprehensible to everyone and ineffective at eliciting any kind of further discussion.

So here's a little bit of explanation/lead-in to the original post that hopefully makes the purpose of it a little bit clearer.

What's an order of magnitude calculation and why would you do one? OK, so sometimes as an engineer (or a scientist) you may know the physical laws a problem has to obey, but not know the precise values of the variables necessary to make an accurate calculation. However, while you may not know the exact value of a variable, that doesn't necessarily mean you know nothing about what values it may take. For example, I may not know the length of your car, but I can say with some confidence that since it's a car, it is of the order of five meters in length. In fact, I would be shocked if your car were less than 1 meter (10^0 m) or more than 10 meters (10^1 m). Note that the exponent (0 or 1) is the order of magnitude of the quantity in parentheses. That's why it's called an order of magnitude calculation... because we estimate the variables based on their typical magnitudes and see what we can learn when we plug numbers of these magnitudes into the physical equations governing the system.

For most any problem (if the estimator has past experience/familiarity with similar problems!) it's possible to go through all the unknown variables and make order of magnitude estimates for their values. If I make such an estimate at say 1 inch, that doesn't typically mean in this type of calculation that I am confident the correct value isn't 1/2 inch or 3 inches... it means I might be surprised if the quantity were .01 inch or 100 inches (if estimating to within two orders of magnitude) or .1 inch or 10 inches (if estimating to within an order of magnitude).

I also pay attention to how closely I think I am able to estimate the variables, and consider the possible impact of the uncertainties when interpreting the outcome of the calculation. When estimates of a variable are known to a very close magnitude, then that tends to give much more useful results than when variables (particularly important variables) cannot be estimated closely at all.

Below I made a very rough calculation of how I thought the accuracy of a bullet might be impacted 100 yards down range given a small base edge defect that resulted in asymmetric venting of gases at bullet departure from the crown. I DO NOT believe that this is a precise calculation by any means... it's not even tied down to a specific load, defect geometry, bullet shape, etc... but I do believe the values I've plugged in are ballpark reasonable.

So what can you really learn from such a calculation? Well, that depends on how much you believe the estimates for the variables, etc. Sometimes, all you learn is that you cannot discount a hypothesis on the basis of such an approximate calculation... Sometimes you learn a hypothesis is totally absurd. Sometimes it becomes obvious that one thing must be much less important than another, just because of the relative magnitudes of the variables involved and the nature of the equations. Sometimes, you learn something about the relative magnitude of a parameter relative to another that sheds light on the behavior of the system.

I think the below calculation did this for me with regards to the magnitude of lateral velocity component that's necessary to open up group size at 100 yards, as compared to the bullet down range velocity component. The relative magnitude is (1500 fps ~= to 10^3, and .2 fps ~= 10^-1, so we're talking about a difference in magnitude of 4 (a factor of 10,000))! So it only takes a relatively tiny (10^-4) variation in the lateral velocity component as compared to the overall speed of the bullet to result in groups opening up.

Other than that, I think the calculation shows that small base edge defects (when determined to be present) are not obviously an unlikely cause for bad accuracy... neither does it prove they are definitely the culprit when present though -- you really need more confidence in the estimates for the variables than I believe I can make without further work.

You should also realise that you should only have as much confidence in the conclusions of this sort of analysis as you have in the ability of the engineer to reasonably estimate the variables. If the engineer can't get the variable estimates in the same state as the ballpark (to use a metaphor), and furthermore they don't recognise the likely magnitude of their error, then you should be very concerned about the reasonableness of any conclusions they draw.

Bret, that's a start, let me know if that makes it clearer what I was doing, below, or if I made things worse.
/EDIT

Ok, so for a while now I've been thinking maybe I should be doing less hand waving on this venting aspect and at least do an order of magnitude calculation (which still involves a lot of hand waving). :)

So here's a first cut, back of the envelope...

At 100 yards a deflection of .5 inches on a bullet travelling 1500 fps requires a lateral velocity of about (.5 inch)/((100 yard*3 ft/yard)/(1500 ft/sec))=2.5 inches per second

So asymmetric venting at the base only has to impart a lateral velocity of .2 fps, which is encouraging! This number at least is very close to the right answer and not much of a guess.

Assume
the bullet weighs ~250 gr (close estimate),
distance over which the asymmetric venting acts is .01 inch, (could be half as large or three or four times as large?)
average pressure is 2000psi (SWAG... has to be much less than base pressure at muzzle due to free expansion over the bullet surface)
action area of pressure is ~ .25 inches (approx. profile area of a 45 cal bullets... This combined with the averaged pressure are likely the most erroneous? Maybe this should be more like a third of profile area for a SWAG).

In this case, the action time for the pressure is .56 microseconds. The force is 500 pounds. The mass is (250 gr/ (7000 gr/pound)*1 pound/32.2 slugs) =.00111 slugs

Therefore acceleration is 500 pounds /(.00111 slugs)=550,550 ft/sec^2

Velocity is 550,550 ft/sec^2*12 inch/ft*.00000056 sec = 3.7 inch/sec

This corresponds to about +1.5 inches growth in group size at 100 yards.

This is in the right ballpark, I guess. So I wouldn't consider gas venting wrecking accuracy to be implausible based on this calculation... But neither does the calc prove anything since it is a rough order of magnitude calculation.

Of course the bore gas is blasting against the base of the bullet for much longer (~300 times as long?) than the venting would occur, and so the magnitude of the average force acting on the bullet could be this much less. Non uniformity of the bullet base could cause asymmetries in the flow that resulted in the .2fps/inch of lateral velocity needed to increase group size, maybe.

Ok, so who is going to crank up the CFD to really get a reasonable estimate of imparted velocity from a small base defect? :)

DrB
07-03-2011, 02:33 AM
Those little extrusions at the base mean nothing if they are even all around.
The problem at the muzzle will still start at the "LAUNCH" as Bass loves to say. He is correct.
Start a boolit out of line with the bore and it is not aligned at the muzzle.

They might not mean nothing if they break away or erode asymetrically or aynchronously in the gas blast... Is how big they are and how well they stay attached related to hardness?

Seems to me that anything asymmetric that can induce that ~.2fps/ (inch group size) lateral velocity can explain accuracy degrading.

Bret4207
07-03-2011, 08:40 AM
Ok, so for a while now I've been thinking maybe I should be doing less hand waving on this venting aspect and at least do an order of magnitude calculation (which still involves a lot of hand waving). :)

So here's a first cut, back of the envelope...

At 100 yards a deflection of .5 inches on a bullet travelling 1500 fps requires a lateral velocity of about (.5 inch)/((100 yard*3 ft/yard)/(1500 ft/sec))=2.5 inches per second

So asymmetric venting at the base only has to impart a lateral velocity of .2 fps, which is encouraging! This number at least is very close to the right answer and not much of a guess.

Assume
the bullet weighs ~250 gr (close estimate),
distance over which the asymmetric venting acts is .01 inch, (could be half as large or three or four times as large?)
average pressure is 2000psi (SWAG... has to be much less than base pressure at muzzle due to free expansion over the bullet surface)
action area of pressure is ~ .25 inches (approx. profile area of a 45 cal bullets... This combined with the averaged pressure are likely the most erroneous? Maybe this should be more like a third of profile area for a SWAG).

In this case, the action time for the pressure is .56 microseconds. The force is 500 pounds. The mass is (250 gr/ (7000 gr/pound)*1 pound/32.2 slugs) =.00111 slugs

Therefore acceleration is 500 pounds /(.00111 slugs)=550,550 ft/sec^2

Velocity is 550,550 ft/sec^2*12 inch/ft*.00000056 sec = 3.7 inch/sec

This corresponds to about +1.5 inches growth in group size at 100 yards.

This is in the right ballpark, I guess. So I wouldn't consider gas venting wrecking accuracy to be implausible based on this calculation... But neither does the calc prove anything since it is a rough order of magnitude calculation.

Of course the bore gas is blasting against the base of the bullet for much longer (~300 times as long?) than the venting would occur, and so the magnitude of the average force acting on the bullet could be this much less. Non uniformity of the bullet base could cause asymmetries in the flow that resulted in the .2fps/inch of lateral velocity needed to increase group size, maybe.

Ok, so who is going to crank up the CFD to really get a reasonable estimate of imparted velocity from a small base defect? :)

Can you put that in terms a sheep farmer can grasp? I've read it 4 times and still haven't got a clue.

felix
07-03-2011, 09:49 AM
Bret, the good Doc is figuring the size of the retro rocket required to compensate for any directional errors during launch. He is finding out compensation is too minuscule to be cost effective. His disclaimer deals with the obscurity of the actual real-time summation of the erroneous vectors (direction and speed) to work with. His discourse is just thinking out loud. ... felix

SciFiJim
07-03-2011, 12:56 PM
After reading the entire thread, I had to go back and read the first post. It seems that all of this assumes a perfect crown on the barrel. Its been my observation that an imperfect (or buggered) crown can be directly blamed for decreased accuracy. This would lead credence to the OP that "Asymmetric venting during departure" would lead to inaccuracy down range due to an uneven "last push". I think most of us deal more with the fact of an imperfect crown than an imperfect boolit base. Of course the effects are additive. I almost said cumulative, but that is not necessarily true. The effects might cancel each other out. I know we are talking small units of measure here, but time and distance down range allow those errors to grow.


You guys are really pushing the edge of my understanding and I enjoy that. I will continue to follow the thread with interest.

bbqncigars
07-03-2011, 01:26 PM
I know I'm going to re-read this thread a few times to fully appreciate it. I wish Handloader would deal with topics such as this.


Wayne

Doc Highwall
07-03-2011, 01:31 PM
There is nothing like sharing information especially with mathematical and scientific back grounds from our family here at Cast Boolits.

Sometimes it is hard to put it in a layman's language so they can understand it, yet not lose the technical background of math and science for the information provided

Cap'n Morgan
07-03-2011, 02:10 PM
I think you can demonstrate when and to what extent gas jetting at departure can destroy accuracy by baselining an accurate barrel/lead bullet combo for accuracy and then change only the crown at the bore. This would be done by introducing a single small nick at the bore-crown edge that will allow early/asymmetric gas venting during bullet/crown departure.

Don't know about accuracy, but altering the crown can certainly change the point of impact:

http://forums.nitroexpress.com/showflat.php?Cat=0&Number=183746&an=0&page=0#Post183746

How's that for a quick 'n dirty double regulation!

felix
07-03-2011, 02:35 PM
Doc, it's even hard for the scientists, engineers, and supportive technicians to communicate between themselves because of some physical phenomenon being contemplated/analyzed/synthesized is beyond historical recognition, i.e., school books, papers, journals, etc. For example, Newtonian math falls apart at the quantum level and requires a new or modified language. Even then, assurance of a resulting product can be questioned and require the use of actuaries. ... felix

44man
07-03-2011, 04:29 PM
:)

Hey, can you post before/after pictures and particulars for the load? (load recipe, velocity)
It would also be interesting to see the same with the load pushed to accuracy failure (if that can safely be done with yours).

To my way of thinking, if these bullets you looked at were from a "good" load, you shouldn't see: rifling width enlargement, gas cutting, or changes to the plain base edge. The first two could cause variation in muzzle velocity and therefore changes in muzzle location at departure, and all of them could cause asymmetric venting at departure.
Base distortion is minimal from rifling extrusion. I must have melted most of my boolits but found a few. Many of these are fully capable of less then 1" at 100 yards from a revolver.
A perfect crown is important but it seems as if the boolit base only needs to be even at exit. If you destroy the base at launch, bets are off. Same as an off side launch from a chamber out of line with the bore. The base might be good but the boolit is out of balance from being forced sideways through the bore.
I actually see more indentations on gas checks then PB.

DrB
07-04-2011, 12:06 AM
Guys, I'm typing this on my phone, so bear with me till wednesday when I have a real keyboard.

IOU... Definition of jargon, rationale of why an engineer does a ROM calculation when he lacks necessary data, and a narrative explaining, simply, what that calc. post was about.

Thanks 44man for the pictures... Have a couple of thoughts about your picture that I will save till wednesday, but there isn't enough resolution to see much about the base edge of these. A profile picture might also add something...

Bret, anything simple can be made impossible to understand with an incomplete or poor explanation by the author. I'll try to improve the explanation and then we can go from there.

Best regards, DrB

/EDIT: Alright, so I added a lead-in to the calc post on the prior page that should be a good start to satisfying the above. Bret, let me know if it helps/if you have any questions? /EDIT

44man
07-04-2011, 08:24 AM
I have another picture I took some time ago of boolit skid. Forgive my cheap camera, not enough pixels.
The left boolit is from my .45 Vaquero and is engraved perfect.
The right is from my .475 and you can see the skid. It has stopped at the base and this boolit is extremely accurate. By making it just a little harder the skid is reduced. It does not appear to get more accurate by much so the WD, WW metal does good.
By stopping skid at the base there is no gas cutting or leading in the bore.
Metal displacement is higher in the GG's then at the base.
The .45 is shot at 1160 fps and the .475 is 1350 fps. It weighs 420 gr and has more inertia to resist spin.

44man
07-04-2011, 08:54 AM
I wish more fellas would recover boolits and study them. It will tell you your next step for the alloy. The boolit MUST maintain it's original shape.
Skid has to stop at the base band and not ever get wider then the rifling.
I would love to see the 16 to 1 boolits shot at 1400 fps. I never could do it because the incantations said over the gun are voodoo and top secret! :confused::mrgreen:
If the base is clean and sharp at the edges, nothing will harm it unless you shoot silly putty. My PB boolits shoot as good as GC ones. Even in my 45-70 BFR and the .454 at max loads. Only base destruction at launch will ruin exit at the muzzle. But then you no longer have the perfect boolit you spent so much time casting, sorting and seating with care. Why some insist on ruining a boolit before it even leaves the brass is a thing I can't comprehend. A cast boolit recovered should have no more damage then a jacketed bullet.

felix
07-04-2011, 11:45 PM
Shoot them hard enough, Jim, and the bases will go concave. Remember, gas expansion goes along the centerline when the projectile is moving, seeking the least resistance at all times. Picture the DOW bubbles going down the drain. ... felix

44man
07-05-2011, 08:49 AM
Shoot them hard enough, Jim, and the bases will go concave. Remember, gas expansion goes along the centerline when the projectile is moving, seeking the least resistance at all times. Picture the DOW bubbles going down the drain. ... felix
OH, OH, water goes down the drain in different directions depending what side of the equator you live in.
Could we be using the wrong twist directions? :bigsmyl2::bigsmyl2:

felix
07-05-2011, 09:00 AM
Colt likes to think so!!!! ... felix

44man
07-05-2011, 09:15 AM
Shoot them hard enough, Jim, and the bases will go concave. Remember, gas expansion goes along the centerline when the projectile is moving, seeking the least resistance at all times. Picture the DOW bubbles going down the drain. ... felix
Enough with the fun.
That is true and I seem to find that more on a GC but I have not decided if it is caused by the edge of the check being forced back by friction.
All the rifles I have or had were 25-20, 30-30, .35, .44 and 45-70 so I can't talk real high power rifles with cast.
I quit hunting with rifles long ago and sold most to buy revolvers.
I don't see it even up to 1800 fps with my PB but all GC boolits show a center depression.
This is how I like to ruin a base. Shoot the boolits into one hole at 50 yards and have boolits run into each other in the trap. [smilie=s:

felix
07-05-2011, 09:36 AM
I don't see how you can get a better centerline conformance than that! That is very close to a condom configuration. ... felix

Marlin Junky
07-05-2011, 05:20 PM
I have another picture I took some time ago of boolit skid. Forgive my cheap camera, not enough pixels.
The left boolit is from my .45 Vaquero and is engraved perfect.
The right is from my .475 and you can see the skid. It has stopped at the base and this boolit is extremely accurate. By making it just a little harder the skid is reduced. It does not appear to get more accurate by much so the WD, WW metal does good.
By stopping skid at the base there is no gas cutting or leading in the bore.
Metal displacement is higher in the GG's then at the base.
The .45 is shot at 1160 fps and the .475 is 1350 fps. It weighs 420 gr and has more inertia to resist spin.

Could there be something else going on here? It looks like the groove on the leading edge (the ogive) is well formed. Can you rotate the boolit so we can have a better look?

MJ

DrB
07-06-2011, 01:13 AM
Can't you have pretty stout proof in the form of a recovered bullet with engraving marks intact? As long as there isn't evidence of skidding/stripping, you should be able to calculate nominal spin rate.

Engraved groove depth as compared to rifling width should even allow you to calculate a maximum error (minimum spin rate) with a few assumptions? Haven't quite thought this through enough to propose a formula when there is widening of the lands evident.

Nominal spin rate in rotations per minute = (velocity ft/sec)/(twist inch/turn)*(12 inch/ft)*(60 sec/min)

For experimental measurement you could do high speed photography with a marked bullet, measure total reflected light levels from a scene with a marked bullet vs. time, or you might be able to do it acoustically with a symmetrically spooned nose or such. Probably not necessarily cheap but very doable if you had a budget.


Well. So I'm back home and I'll try catching up on some thoughts on this thread.

First, with this one.

As stated before, knowing the spin rate of a bullet is as simple as knowing the muzzle velocity and barrel twist, as long as you have evidence that your bullet isn't skidding in the bore (as indicated by the engraved lands on a recovered bullet being the same circumferential width as the rifling lands). No fancy smancy instrumentation is necessary beyond a chronograph and a recovered bullet evidencing the engraved lands absent skid. The bullet rpm in this case is calculated as above:

1) Nominal spin rate in rotations per minute = (velocity ft/sec)/(twist inch/turn)*(12 inch/ft)*(60 sec/min)

Note also that skid of engraving on a bullets surface may not necessarily mean that the bullet doesn't leave the muzzle at nominal rpm for the muzzle velocity and twist. It is possible for all the evident "skid" of a bore's land engraved on a bullet to occur at some initial or intermediate point in the barrel, after which the rifling holds without further skid. In this case the nominal rpm will be the actual rpm, regardless of stripping.

One variant of this can be when a bullet first engages the rifling. In this case, the rifling will initially engrave just the initial edge of the bullet diameter and this material may easily provide insufficient strength to impart the necessary torque to the rest of the bullet without enlarging the engraved land on this leading part of the bullet. As more and more of the bullet engraves, more contact area is available to impart torque to the bullet, and absent increasing acceleration, the bullet is likely to evidence less skid the further back on the bullet the rifling engraves. I don't see how this "starting skid" would necessarily impact accuracy as long as the bullets were all of fairly consistent hardness and the engraved rifling at the rear of the bullet wasn't enlarged to the point that gas-leakage occurs. I would though, tend to think that engraving of this sort might be indicative of a borderline load, especially as the widening of the lands approaches the base of the bullet. By "borderline load," I mean one for which if the initial acceleration were slightly increased, accuracy would rapidly deteriorate (due to the enlargement of the engraved groove at the base of the bullet).

Next, let's discuss calculating a maximum error in estimated rpm due to bullet skid so as to arrive at a minimum possible bullet rpm. Unless you assume something regarding the acceleration profile of the bullet in the bore, you cannot calculate a non-trivial maximum error (to subtract from the calculated nominal bullet spin rate) on the basis of the width of enlarged engraving. For example, assume the (unrealistic) case of a bullet barely moving down the bore (say 1 fps), then slammed in the last inch of travel up to a muzzle velocity of 1500 fps. In this hypothetical case in a 1 turn in 20 inch barrel, the engraved rifling would skid 18 degrees.

Nominal estimated rpm would be (from 1): (1500 fps)/(20 inch/turn)*(12 inch/ft)*(60 sec/min)=54,000 rpm

Actual rpm would be (also from 1, but using 1 fps for velocity): (1 fps)/(20 inch/turn)*(12 inch/ft)*(60 sec/min)=36 rpm

As you can see, if you are unable to make any assumption about the bullet acceleration profile the largest theoretical error is 100% (EDIT: IF THE BULLET IS SKIDDING!)

<TO BE CONTINUED> (EDIT: I think there are some simple and reasonable assumptions we can make to get an estimate of bullet rpm error in the event of skidding (without stripping)...)

DrB
07-06-2011, 01:15 AM
Enough with the fun.
That is true and I seem to find that more on a GC but I have not decided if it is caused by the edge of the check being forced back by friction.
All the rifles I have or had were 25-20, 30-30, .35, .44 and 45-70 so I can't talk real high power rifles with cast.
I quit hunting with rifles long ago and sold most to buy revolvers.
I don't see it even up to 1800 fps with my PB but all GC boolits show a center depression.
This is how I like to ruin a base. Shoot the boolits into one hole at 50 yards and have boolits run into each other in the trap. [smilie=s:

Hah! Robinhood, .429 style! :)

DrB
07-06-2011, 01:21 AM
I have another picture I took some time ago of boolit skid. Forgive my cheap camera, not enough pixels.
The left boolit is from my .45 Vaquero and is engraved perfect.
The right is from my .475 and you can see the skid. It has stopped at the base and this boolit is extremely accurate. By making it just a little harder the skid is reduced. It does not appear to get more accurate by much so the WD, WW metal does good.
By stopping skid at the base there is no gas cutting or leading in the bore.
Metal displacement is higher in the GG's then at the base.
The .45 is shot at 1160 fps and the .475 is 1350 fps. It weighs 420 gr and has more inertia to resist spin.

Neat...

So in this picture, the rifling is forcing the top of the bullet to move (rotate) from left to right. The right side of the engraved groove is taking compression, and the rifling is scraping this material to right during skid. Thus the right side of this engraved land looks sharp while the left is pushed down and washed out.

So I'm thinking that if this is what it appears to be in the photo (wide at nose, narrow at base), then it must be from skidding during the initial engraving. Initially, the rifling has little bullet metal to grab/less contact area, then more and more as the bullet engraves the further it enters the bore (simultaneously spinning up to match the velocity/rifling twist rate rpm), such that before the bullet becomes fully engraved it is spinning at the rifling twist rate.

That said, it kindof looks like from the photo there may have still been some skid after the base was in the rifling?

felix
07-06-2011, 12:40 PM
The only way to get the RPM is through photography, as was already decided. A projectile with various acceleration points will not be consistent enough to be anywhere close to being accurate enough to be "reliable", either within the barrel or not (negative acceleration for sure). ... felix

44man
07-06-2011, 01:39 PM
Extremely good postulations given.
I will be bouncing back and forth. I am making beer and sparging, getting ready to boil.
The 420 gr boolit has base engraving at a perfect fit to the rifling. Had I air cooled, it would be exceeded and create gas channels. Not good!
True that initial skid means little and the boolit will still spin as required.
More skid would cause fliers and leading. That would require a harder boolit. To say a softer boolit would correct leading is wrong.
If I was to shoot this boolit faster, it needs even harder lead.
This is why it is important to study recovered boolits and not guess. That little piece of lead has a thousand stories.
I wish I had a dollar for every one that said the boolits need to be softer without knowing all the facts! [smilie=l:

DrB
07-06-2011, 11:54 PM
The only way to get the RPM is through photography, as was already decided. A projectile with various acceleration points will not be consistent enough to be anywhere close to being accurate enough to be "reliable", either within the barrel or not (negative acceleration for sure). ... felix

What?!!

Who decided that? And did they consult with reality before making that decision? :)

I suggested that if you wanted direct measurement, you could use photography to measure bullet rpm. There are other ways, but with photography is way, way within the state of the art. Lord, I think folks have been taking pictures of bullets in flight of one kind or another at least back to the time of Ernst Mach! Well, Mach took pictures of the shock waves around a supersonic bullet (hence "Mach Number" and "Mach Wave")... not sure if he took pictures of the surface of a bullet itself. But heck, 18 years ago as an undergraduate, I did some tests with Johnson's HIRL in which we took stop motion pictures of a hypervelocity impact of a bullet on a target at around ~7km/sec (~23,000 fps).

In prior posts I also pointed out that if the bullet wasn't skidding (as evidenced by a recovered bullet), it was trivial to get spin using a chrony and barrel twist.

Absent evidence of skid, rpm can be calculated as the product of muzzle velocity and twist (with appropriate units). Absent skid, the rpm estimate should be as good as the accuracy of the velocity and twist measurements.

What is "various acceleration points" supposed to mean?

"Negative acceleration for sure"?

Felix, I'm not understanding what it is you are trying to say here... all I'm hearing is pops and whistles on this end. :???:

DrB
07-07-2011, 01:43 AM
I think bullet base squareness to the bore has been mentioned in the context of accuracy a lot, both in this thread and in others, and I've been thinking about it in the context of this thread.

I thought I'd do a quick calculation as to what the lateral force is on a bullet due solely to the bullet base angle and base gas pressure. Note that this is entirely aside from the forces resulting from early venting that create pressure on the ventward side of the bullet. As it happens, these two forces should act in concert to give the bullet a sideways shove in the direction the bullet base is angled towards.

So, definition of variables is as follows:
A = Bullet base area
F = Total force on bullet base due to gas pressure
Fl = Lateral Force component (shoves the bullet sideways)
C = Caliber (in inches) of the firearm
P = Bullet base pressure at the muzzle (this can be estimated from any ballistics program that allows you to vary barrel length and see the change in expected muzzle velocity)
Br = Bullet base angle off normal in radians
Bo = Bullet base angle off normal in degrees
pi = the ratio defined by a circles circumference over it's diameter
ksi = 1000 psi

A = pi/4 * C^2
F = A * P = pi/4 * C^2 * P
Fl = pi/4 * C^2 * P * SIN(Br)

So, by the small angle approximation,

Fl = pi/4 * C^2 * P * Br
Br = Bo * pi/180, so substituting, we have
Fl = pi^2 / 720 * C^2 * P * Bo

So, the bottom line is, per degree that the base is off square, and per 1000 psi of bullet base pressure, the lateral force may be calculated as:

Fl = C^2 * 13.71 lbf / per ksi of bullet base pressure / per deg that bullet is off-square

So, let's do a sample calculation.

If you are shooting about a .452 bullet, and base pressure on the bullet at the muzzle is 5,000 psi (= 5ksi), and the bullet base is just 2 degrees out of plumb, then the lateral force on the bullet is:

Fl = (.452)^2 * 13.71 lbf * (5 ksi) * (2 deg) = 28 lbf

That's actually a pretty significant force. Consider that in our earlier sample calculation with a bullet going 1500 fps, only .2 fps lateral velocity was needed to open up group size at 100 yards by 1 inch. Also, keep in mind that a bullet of say 300 grains is only .00133 slugs... which means it doesn't take a lot of momentum change to give a 1/1000 of a slug bullet a .2 fps lateral kick.

So how long would the average base pressure have to stay at 5 ksi (averaged over the time):

Let's start by finding the acceleration the bullet experiences...
acceleration = force / mass = 28 lbf / .00133 slugs = 21,053 feet/sec^2 (that's 653 gravities!)

Next we find the action time required to impart .2 fps...
time = velocity/acceleration = .2 fps/ 21,053 = 9.4 microseconds

At 1500 fps, this corresponds to a distance of 14 thousandths of an inch. With a bore full of high pressure gases venting, maintaining the base pressure close to the sealed muzzle value for just 9.4 microseconds and a vent gap of .014 inches shouldn't be hard.


So, what does all the above mean? Well, it seems pretty clear to me that a VERY slightly off-square base can result in a large degradation in accuracy, solely due to the lateral shove that results from an off-square base. By the time you can feel an off-plumb bullet base, you are probably experiencing some degradation. By the time you can see an off-plumb base, accuracy may well be totally gone. A degree of angle is a very small thing to notice on such a short surface...

DrB
07-07-2011, 02:01 AM
Shoot them hard enough, Jim, and the bases will go concave. Remember, gas expansion goes along the centerline when the projectile is moving, seeking the least resistance at all times. Picture the DOW bubbles going down the drain. ... felix

That seems very dubious to me. We're talking about thousands of psi of gas pressure, at high temperatures. The speed of sound in the gas is going to be REALLY high, and the mean free path is going to be REALLY small. The time required to communicate a pressure differential across the base of the bullet (and for the gas to move to compensate) is going to be TINY. I think the best way to give your notion a reality check is probably to calculate the mean velocity of molecules in the gas and the mean free path, which I'm pretty sure would show you that the gas is going to be in quasi-equilibrium across the base of the bullet (base pressure should be relatively uniform). Consider that the gas is able to expand down the bore to maintain relatively high bullet base pressures tens of calibers away (even with the bullet racing away) -- does it really seem likely to you that a significant pressure gradient will be maintained across the base of the bullet (just half a caliber of distance)?

Have you ever seen a numerical or theoretical calculation that suggests that what you are saying is physically possible, or is this just a hypothesis based on the cupping in a gas check base?

I've never seen one of these cupped bullet bases, maybe there's something else about them that makes you think it's gas pressure differential across the base of the bullet and not something else?

As someone else mentioned, if this only happens with gas checked and not PB bullets, in identical loads, then it seems to me that the gas pressure hypotheses would be discredited by that...

DrB
07-07-2011, 03:55 AM
Don't know about accuracy, but altering the crown can certainly change the point of impact:

http://forums.nitroexpress.com/showflat.php?Cat=0&Number=183746&an=0&page=0#Post183746

How's that for a quick 'n dirty double regulation!

Ah, great!

So, if one were to accept that a comparable size/geometry of defect on the bullet base edge can have very similar effect as when on a crown, then it becomes obvious that whereas a defect on the crown might change point of impact, defects on the bases of bullets will change accuracy.

This is because the clocking of the defect on the bullet at departure from the crown is likely to be unbiased to any particular direction. Therefore the lateral shove that results from the gases may occur in any direction, and the bullet drift during flight down range from this shove can be in any direction. In other words, on one shot bullet base edge defects or base edge angle may result in the bullet flying high, the next time right, the next time one way, and then another.

To maximize accuracy we want everything to be as repeatable as possible. If we do not control, or have no means to control, the clocking of a bullet base edge defect at departure from the crown, then we cannot repeat the resulting shift in point of impact.

EDIT: I tool another look at the thread referenced on the crown grinding being done on some sabatti rifles. Sure looks like a LOT of material removed. Just to be clear, I kind of doubt anyone here is shooting bullets with comparable defects (or expecting good accuracy if they are).

44man
07-07-2011, 08:57 AM
It is still a habit of many BPCR shooters to chamber rounds in exactly the same position and to seat boolits in some alignment to case orientation. Probably a holdover from when molds were cut by hand in a vise.
With modern dies and molds it is a waste of time.
I had a large chunk of aircraft aluminum that I sawed into blocks and milled. I have no way to hold the blocks to the table so I use my vise. The very hardest job is to try and get 90* all around.
Once the blocks are clamped into my mold vise, the top is milled first so it does not matter if a side is a degree off. Boolits will be 90* to the mold top.
My molds are not pretty but I have to wonder if that extra step is why my boolits are so accurate???
There are stories of fellas milling the tops of molds to remove BB's, or change weights. Just how does one align blocks to the center line of the cavities? I am far from being a machinist so I can't understand how it could be done if the blocks are not perfect.
Take a good machinist square and check the outside of your blocks once. Lee molds are real funny! :grin:
Then a few lap mold tops and sprue plates trying to make them flat. Can't happen! :kidding:

Cap'n Morgan
07-07-2011, 10:55 AM
Ah, great!

So, if one were to accept that a comparable size/geometry of defect on the bullet base edge can have very similar effect as when on a crown, then it becomes obvious that whereas a defect on the crown might change point of impact, defects on the bases of bullets will change accuracy.



One way to check how much a defect base will interfer with accuracy would be shooting deliberately mangled boolits through a ported barrel where the porting holes could be shut on and off. If your thinking is right (and I believe it is) the deformed base bullets should shoot significantly better when the pressure is bled off while the bullet is still in the barrel.

I have ported several barrels to reduce recoil. I use several rows of small holes drilled between the lands, and following the twist of the rifling - typically 3, 4 or 6 rows with 5 to 8 holes each - all depending of the number of grooves in the barrel. Not that hard to do if you have a CNC with a controlled 4th axis. It would be a simple matter to make a sleeve to slip over the muzzle to block the holes.

I'm not sure how the holes would react to high speed cast bullets, though. I have only tried some subsonic flat base in 6.5 x 55 and they didn't cause any problems. But I would expect some minor gas cutting to the base unless gas checks is used.

felix
07-07-2011, 10:59 AM
A very cool idea, Cap'n. ... felix

DrB
07-07-2011, 02:07 PM
One way to check how much a defect base will interfer with accuracy would be shooting deliberately mangled boolits through a ported barrel where the porting holes could be shut on and off. If your thinking is right (and I believe it is) the deformed base bullets should shoot significantly better when the pressure is bled off while the bullet is still in the barrel.

CM, one good experiment is often worth a pile of calculations, and years of lip flapping. Reality is. That's one of the reasons I think Ackley is still worthy of respect today. He did a number of experiments with firearms to demonstrate the failings of various lines of thinking at the time.

Yes, I had nearly the same thought in one of the earlier posts, but I thought to do the accuracy test before and after porting (and deburring of the barrel). It may be more repeatable your way, don't know. You've certainly got the opportunity to go back and forth your way. There's the potential for a lot of reaction force on a sleeve if it vents more one way than another, and you might rupture it if it is too thin. I would think though you'd be fine if you made it relatively thick, with a tight id to barrel od tolerance, and pin, bolted or threaded it. And lots of grease between the barrel and sleeve plus a flat or groove to help you drift it back off if it fouls to the barrel.

I don't know what the holes will do to the bullet. On the one hand I think folks shoot gas operated guns with well fitted boolits with negligible leading. On the other hand I've come across suppresor manufacturers saying they avoid small holes in integrally suppressed guns due to excessive fouling with boolits (large holes supposedly didn't foul so bad).

If you ever decide to undertake such an experiment, let me know and i'll pitch in (if it would be of help to you) for one of those green mountain gunsmithing special barrels.



I'm not sure how the holes would react to high speed cast bullets, though. I have only tried some subsonic flat base in 6.5 x 55 and they didn't cause any problems. But I would expect some minor gas cutting to the base unless gas checks is used.

DrB
07-08-2011, 01:32 PM
Base distortion is minimal from rifling extrusion. I must have melted most of my boolits but found a few. Many of these are fully capable of less then 1" at 100 yards from a revolver.
A perfect crown is important but it seems as if the boolit base only needs to be even at exit. If you destroy the base at launch, bets are off. Same as an off side launch from a chamber out of line with the bore. The base might be good but the boolit is out of balance from being forced sideways through the bore.
I actually see more indentations on gas checks then PB.

So the two thoughts I mentioned regarding your pictures were that first of all, it's interesting how well the check conforms to the bullet base. It seems like you can see every bit of the sprue impression through the copper check. Given the pressure with which the check is swaged to the base that isn't all that surprising, though.

Second, the edge of the gas check appears to have been both stiff enough and supported enough that it doesn't appear to have deformed much at the edge from base pressure, but has kept what looks like a nice even radius. This would be a good thing from a venting at departure standpoint. I'll bet you could file a noticeable notch in the base edge of a cast bullet, seat a gc, shoot, and recover it, and the gc would maintain a pretty good edge in spite of the underlying defect. If you then shot some modified this way with and without gas checks i'll bet it would show the majority of impact to accuracy from the removed material was from gas flow, not unbalancing of the bullet (the gas check would shield the defect from the flow).

It also makes me wonder if a short ring gas check (with no base) would stay on the bullet without the huge base gas pressure force which tends to hold on a standard gas check. I had wondered earlier about testing the base edge venting effect of a short copper ring, but I wonder now if it would be able to stay on or if it would leak between the check and bullet base.

felix
07-08-2011, 01:55 PM
Naked based condoms will stick to the bore during some instances of firing, allowing the lead to squirt through as a naked condom. Happens when 30-06 ammo is loaded full tilt with a 150-152 grainer military bullet sawed off about 1/8 inch and then hollow pointed. Might not happen if a drilled out gas check was severely crimped on. ... felix

44man
07-08-2011, 03:11 PM
Naked based condoms will stick to the bore during some instances of firing, allowing the lead to squirt through as a naked condom. Happens when 30-06 ammo is loaded full tilt with a 150-152 grainer military bullet sawed off about 1/8 inch and then hollow pointed. Might not happen if a drilled out gas check was severely crimped on. ... felix
Darn, I just printed a bunch and a thunderstorm shut the juice off for a second.
It could be a disaster if the ring is pulled off and sticks in the bore. I still think the edge of a gas check is pulled back from friction and gives the appearance of center pressure. That would pull a ring of copper.
The sprue mark shown means the copper just formed around it and did not flatten it.
Think of the thin skinned bullets shot out of the .357 max and the .454 that the cores were driven out of leaving the jackets in the bore. That is why they made magnum bullets. Copper has more friction then lead.

felix
07-08-2011, 03:52 PM
I still think the edge of a gas check is pulled back from friction and gives the appearance of center pressure.

Yes, that too. The hollow base appears with no gas check as well. I first saw it first shooting 41 maggies quite hard. ... felix

DrB
07-08-2011, 09:13 PM
I'd REALLY like to see some checked and pb examples of base cupping. Reality is, and I've never held one of these in my hand and seen what it looks like.

Felix (or anyone), could you please recommend a bullet hardness/weight/powder load in 357 that you have seen produce base cupping, if you have for that cartridge? I'm better equipped for experimenting in 357 than I am 44, and I have no 45 lc in the stable right now.

Best regards,
DrB



EDIT 7/12/2011: No one has a load they can suggest that they think will produce cupped base bullets with and without gaschecks? Or just not in .357 Magnum?

DrB
07-12-2011, 05:35 PM
In a previous post in this thread I derived an estimate for the lateral shove given by a non-flat gas check, as a function of the angle of the gas check (or pb) from normal. The problem with relating this to the gas checked bullets sitting on yer bench is that it's not so easy to measure angle directly... it's much easier for most of us to measure length.

I was having some difficulty with some poorly fitting gas checks last night, so I thought I'd do a quick calculation to estimate the angle a gas check was off flat based on a bullet length measurement from the shortest to the longest lengths of the bullet from nose to tail (easiest to measure with a wide meplat or cyclindrical bullet, probably).

Symbols:
Bo = angle off normal in degrees (zero for a perfectly normal base)
Brad = " " in radians
C = Caliber of the bullet
l2 = longest length, nose-to-base, of the bullet
l1 = shortest length, nose-to-base, of the bullet
pi = the ratio of a circles circumference to its diameter

(l2-l1) = C*sin(Brad) ~= C*Brad = C* Bo * pi/180

So, therefore:
Bo = (l2-l1)*180/(C*pi)

Sample calculation:
Let's say we have a .225 diameter bullet, and prior to seating the gas check the nose to length measurement was uniform to within a thousandths, but after seating the gas check there was .005" difference.

This would give us:

Bo = (.475-.470)*180/(.225*3.14159) = 1.27 degrees


Best regards,
DrB

jandbn
07-13-2011, 09:26 PM
DrB,

Kudos to you for applying the knowledge you have and sharing it with us! If you wrote articles for one of the gun magazines with the in-depth writing you have done in this thread, I would subscribe (dropped my subscriptions 20 years ago). This is a very intriguing topic to say the least even though I forgot most of my high school Physics and Geometry from 35 years ago. I can grasp the concept but it is still mind boggling for me and I would also assume for others too as some of the subject matter is very “heavy”.

My question is not directly related to the trailing edge and I don’t want to hijack this thread, but I would like to hear your and others opinions with regards 44Man’s reference to both BPCR shooters chambering rounds in the exact same position and molds where the sprue plate does not cut the boolit base perpendicular to centerline. As much as I would like to test this my self (not equipped at this time), would a batch cartridges with boolits of identical exaggerated out-of-square bases where the base was oriented the same on firing for each cartridge, hit the same POI or would the group just open up? Maybe this was already mentioned in this or another thread or someone elsewhere has already tested this and I just haven’t found reference to it yet? My ‘imagination’ says that a person could test this with a little time and effort if there were some tools available. The test, if it already has not been done, could be accomplished via a “control load” and a load where a boolit’s base angle has been purposely modified.

Instead of trimming the case mouth with a case trimming die and a file, make (with a lathe) a couple devises similar to them except the dies purpose would be for trimming the base of a boolit. The boolit would be inserted nose first in to an adjustable depth hole (tap threads in bottom of the die for bolt) where the boolit can just be inserted via very light finger pressure. One die’s top would have the “exaggerated” angle (grind or cut the angle) and the other die’s top would be perpendicular to the hole the boolit is inserted into. For the “variable” batch of boolits, lightly file each base to the angle (how much angle?) of the angled top die (leaving a little bit of the base not filed for orientation use). Then to make the other batch of “control” boolits the same weight as the angled base boolits, weight a couple of the angled base boolits for an average weight and then trim off the appropriate weight for the quasi-control boolits so that the only “variable” to the loaded cartridges are the boolit bases. Then load and shoot for groups using the BPCR orientation technique to see what comes of it.

With what I’ve garnered so far in this thread, my assumption (everybody knows what assuming does…) is that that even though the batch of cartridges with the angled boolit bases are oriented the same upon firing, the group will open up instead of shift POI. Then again, there is the article Cap’n Morgan referenced about the "fine tune regulated" Sabatti rifles where the rifling in the crown was removed to ‘steer’ the bullets. Angling the base of a boolit is kinda-sorta-similar to a "fine tune regulated” crown?

felix
07-14-2011, 12:09 AM
Doc, use WW or softer (air dried) with nothing slower than 231. I use 700X because my lot is even faster than my lot of BE. The wholesaler sold it to me cheap because he knew the lot was faster than spec somehow, someway. Use FULL load, 35K CUP or thereabouts. If the boolit does not fit correctly, expect tons of leading. Have a film of oil in the cylinder and barrel before shooting. Have some steel or copper wool handy. Steel wool is soft enough because it has little or no carbon content compared to most gun metal. Make sure bases are flat before firing. Finger rub test is good enough, before and after firing. ... felix

Bret4207
07-14-2011, 07:40 AM
So, what does all the above mean? Well, it seems pretty clear to me that a VERY slightly off-square base can result in a large degradation in accuracy, solely due to the lateral shove that results from an off-square base. By the time you can feel an off-plumb bullet base, you are probably experiencing some degradation. By the time you can see an off-plumb base, accuracy may well be totally gone. A degree of angle is a very small thing to notice on such a short surface...

So, being rather the dullard mathematically, did you add alignment issues into the mix? To my mind there is very little difference in the end result between a poorly aligned boolit and an unsquare base or crown. In fact, I suppose it's possible they'd occasionally cancel each other out. Variables add up, that I don't think we can fight.

Thought's?

felix
07-14-2011, 01:22 PM
That, Bret, causes more havoc than anything. Can't tell what's going on. Once detected, it time to relegate that ammo to charging beer cans. Make the ammo mo'betta' next time. Been there, done that. ... felix

Bret4207
07-14-2011, 07:32 PM
Thanks Uncle Felix, at least I don't feel like a complete 'tard for asking!

DrB
07-15-2011, 01:51 PM
Kudos to you for applying the knowledge you have and sharing it with us! If you wrote articles for one of the gun magazines with the in-depth writing you have done in this thread, I would subscribe (dropped my subscriptions 20 years ago). This is a very intriguing topic to say the least even though I forgot most of my high school Physics and Geometry from 35 years ago. I can grasp the concept but it is still mind boggling for me and I would also assume for others too as some of the subject matter is very “heavy”.

Thanks, jandbn. It's kind of you to say that...

OTOH, the great thing about this site is the wealth of observations that all the participants have accumulated over the years. We can all benefit by sharing the results of our occasional experiment/range trial. The forum of today actually has a great deal in common with the early scientific journals (where letters amongst interested individuals were exchanged and published). On the other hand, the latest engineering computational tools are out of reach of the typical reloader/enthusiast. It'd be nice if we could get some basic results updated... there are a lot of informative articles that could be published.



My question is not directly related to the trailing edge and I don’t want to hijack this thread, but I would like to hear your and others opinions with regards 44Man’s reference to both BPCR shooters chambering rounds in the exact same position and molds where the sprue plate does not cut the boolit base perpendicular to centerline. As much as I would like to test this my self (not equipped at this time), would a batch cartridges with boolits of identical exaggerated out-of-square bases where the base was oriented the same on firing for each cartridge, hit the same POI or would the group just open up? Maybe this was already mentioned in this or another thread or someone elsewhere has already tested this and I just haven’t found reference to it yet? My ‘imagination’ says that a person could test this with a little time and effort if there were some tools available. The test, if it already has not been done, could be accomplished via a “control load” and a load where a boolit’s base angle has been purposely modified.


As long as the bullets maintain their clocking on exit from the muzzle (no skid, or consistent skid of the bullet in the rifling), then yes, you'd expect the group to be deflected in the direction the base was pointed towards upon exit from the muzzle. With an indexed ramrod and tight fitting patch, I'd suppose you should even be able to predict what direction that should be by checking rotation from chamber to crown.

Now, you would have to remember that group size is due to a number of different sources and not just the lateral shove a bullet may get upon departure from the crown... so if you had a sufficient enough angle on the base you would expect the group to be steered. It probably would be most observable for a given group of off-flat projectiles if you ran some clocked one way and another batch grouped 180 degrees opposite, shooting into two different targets.



Instead of trimming the case mouth with a case trimming die and a file, make (with a lathe) a couple devises similar to them except the dies purpose would be for trimming the base of a boolit. The boolit would be inserted nose first in to an adjustable depth hole (tap threads in bottom of the die for bolt) where the boolit can just be inserted via very light finger pressure. One die’s top would have the “exaggerated” angle (grind or cut the angle) and the other die’s top would be perpendicular to the hole the boolit is inserted into. For the “variable” batch of boolits, lightly file each base to the angle (how much angle?) of the angled top die (leaving a little bit of the base not filed for orientation use). Then to make the other batch of “control” boolits the same weight as the angled base boolits, weight a couple of the angled base boolits for an average weight and then trim off the appropriate weight for the quasi-control boolits so that the only “variable” to the loaded cartridges are the boolit bases. Then load and shoot for groups using the BPCR orientation technique to see what comes of it.


Another way to do this would be to take a naturally varying population (collection) of bullets and characterize their out-of-flatness, clock each one to a known orientation, and shoot each one into it's own target. You'd want to do this at relatively close range to reduce the impact of wind, and you'd want to randomize or distribute the unflatness in the order of shots so as to average out the effect of barrel heating, fouling, etc. In this study it probably would also make sense to clock half of the bullets 180 degrees in opposition to the rest to maximize the round-to-round Point of Impact (POI) variability due to base non-flatness. You then would measure the POI on each target for each bullet, and using the POI with the measured base angle you could do a regression analysis (least squares fit) and see how good a correlation there was.



With what I’ve garnered so far in this thread, my assumption (everybody knows what assuming does…) is that that even though the batch of cartridges with the angled boolit bases are oriented the same upon firing, the group will open up instead of shift POI. Then again, there is the article Cap’n Morgan referenced about the "fine tune regulated" Sabatti rifles where the rifling in the crown was removed to ‘steer’ the bullets. Angling the base of a boolit is kinda-sorta-similar to a "fine tune regulated” crown?

I would not expect the group to necessarily open up if the base were angled the same each time. I think that usually this sort of effect is not controlled for by most (we don't mark and "clock" our ammo) and thus the angle of clocking of the cartridge in the chamber is randomized, and hence the effect on target is randomized such that the group opens up instead of shifting.

RE filling of a crown vs. bullet base: Yes... in that both should result in a lateral shove to the bullet. I would speculate that if you had an angled based boolit in a a load that didn't skid them down the bore, that you should be able to "regulate" a rifle by controlling the clocking of the bullet base at muzzle departure (by clocking the bullet on insertion to the chamber).


Actually, I was thinking of doing the same experiment (sort of) as the one you've suggested, but with some poorly fitting gas checks on some 22 caliber boolits. I suspected that they were sitting to varying degrees of angle as the gas check was too narrow to fully accept the boolit shank, and marks on the bullet seemed to indicate they were not seating concentrically. So I had high plans for measuring a bunch of them, marking them individually, and then clocking and labelling each of the loaded rounds. Before starting I measured a few of the unchecked boolits and round that while the bases appeared to be nicely flat, the mold halves were apparently about .002-.003" offset. Gas-checked brethren came in with about the same variation in OAL of the boolit, so as near as I can tell the checks are ending up flat.

So for me, it looks like that experiment will have to wait.

Something I'm curious about that maybe one of our machinists here could help with is better ideas for measuring the angle of a boolit base. Absent making a mold, slicing it, and putting it on a comparator, or some pretty expensive CNC digital measuring equipment I don't know how exactly you'd go about doing it (unless the boolit started out consistent in nose flatness/OAL and you could measure the base to nose variation in OAL and get an angle thereby).

Best regards,
DrB

DrB
07-15-2011, 01:54 PM
Doc, use WW or softer (air dried) with nothing slower than 231. I use 700X because my lot is even faster than my lot of BE. The wholesaler sold it to me cheap because he knew the lot was faster than spec somehow, someway. Use FULL load, 35K CUP or thereabouts. If the boolit does not fit correctly, expect tons of leading. Have a film of oil in the cylinder and barrel before shooting. Have some steel or copper wool handy. Steel wool is soft enough because it has little or no carbon content compared to most gun metal. Make sure bases are flat before firing. Finger rub test is good enough, before and after firing. ... felix

Thanks, felix. Any suggested weights for boolits (would you expect this to be more noticeable with a light for caliber bullet where in bore velocity/acceleration was higher)?

DrB
07-15-2011, 01:58 PM
So, being rather the dullard mathematically, did you add alignment issues into the mix? To my mind there is very little difference in the end result between a poorly aligned boolit and an unsquare base or crown. In fact, I suppose it's possible they'd occasionally cancel each other out. Variables add up, that I don't think we can fight.

Thought's?

No, Bret -- to my way of thinking, all that matters is the geometry of the base at departure (as far as the lateral shove a bullet gets from the escaping bore gases).

If the base is off flat because of how the boolit entered the forcing cone/rifling, that should cause just as much of a shove per degree of off flatness as a bullet base that starts off flat (assuming that the base of a bullet askew from a forcing cone entry is flat and not some other geometry -- seems reasonable enough to me as a first cut).

EDIT/: Now, one thing I'm not sure of is what happens with a gas-check that's not flat, initially. There's some serious pressure that will tend to flatten it against a bullets base, but I don't know if the thicker material of the edge of the check will fully level to the bullet base, and if it will do so all the time/uniformly. Folks have talked about on this forum before that bullets with initially unflat checks tend to be poorly accurate, and that makes sense to me. But I do wonder how much the edge gets ironed down vs. the flat of the check...

felix
07-15-2011, 02:20 PM
Heavier boolits work mo'betta', Doc, for the experiments you are contemplating. ... felix

DrB
07-15-2011, 11:11 PM
Thanks, Felix.

303Guy
07-23-2011, 07:53 PM
I've had plenty examples of cupped bases (I use a 'test tube' in my shed). I put it down to extrusion of the boolit as it enters the bore after being 'swaged' down through the tapered throat and leade. The cupping would be caused by the pressure exerted on the base preventing even rearward metal displacement. I take the view that if there is lead displacement causing cupping then there is a chance of that cupping being uneven. For that reason I make my boolits with a small rebate.

I have a suspician that a damaged or uneven boolit base is also going to have an effect boolit flight as in causing some degree of yaw.

DrB
08-02-2011, 03:47 PM
That seems very dubious to me. We're talking about thousands of psi of gas pressure, at high temperatures. The speed of sound in the gas is going to be REALLY high, and the mean free path is going to be REALLY small. The time required to communicate a pressure differential across the base of the bullet (and for the gas to move to compensate) is going to be TINY. <snip>

Ok, so I was looking at one of 303Guy's pictures he posted last night of recovered patched and unpatched loads showing base cupping, and had an idea on the subject of base cupping. Other than cream of wheat filler in the cupped bullet base, as I understand it, these loads were identical paper patched bullets loads. In the first picture on the left, the bullet on the left shows cupping with a roughened peened base, the bullet on the right does not. The picture on the right, below shows a closer/better contrast picture of the cupped base bullet from the first picture.


http://i388.photobucket.com/albums/oo327/303Guy/DSCF5572_edited-1.jpghttp://i388.photobucket.com/albums/oo327/303Guy/DSCF5574_edited.jpg

Felix, et al, I stand by what I said about the mean free path in the gas being tiny, and unlikely to effect base cupping due to pressure... but what I wasn't considering is that the flow at the base of the bullet is generally NOT JUST A GAS. :groner: :) Most loads are not equivalent to a "pre-burned propellant" gun (loads like light bullseye behind a very heavy bullet probably come closest to a PBP gun). That is, the flow in the barrel is a mixed phase flow comprised of both gas and solid particles. In 303Guy's example, much of the solid component is comprised of cream of wheat. In other examples, it could be comprised of burning powder particles.

I suspect that what these larger particles effectively do, when high in concentration in the flow, is decrease the effective mean random velocity of the flow and increase the mean free path. I would expect there to be a ring vortex of particles following the base of the bullet, with the particles travelling fastest up the center of the bore, impinging on the bullet base, travelling outward towards the rifling, and then slowing down and travelling backwards relative to the base of the bullet, then turning back inward under the influence of the burning gases expanding down the bore and the centerline vacuum created by the vortex.

So the idea is that because of the influence of the LARGE particles in the flow of gas, and the boundary condition at the bore/bullet trailing edge (which is travelling quickly backwards relative to the bullet fixed frame of reference), the effective pressure on the edge of the bullet is substantially reduced relative to that at the centerline of the bullet. The effect is not due to "gas pressure" so much as the reaction pressure on the bullet base of the solid component of the mixed phase flow. Gas pressure is reduced at this boundary, but it is because of the effect of entrainment of the gas by the larger particles, which are in turn greatly retarded by the barrel/bore boundary condition.

In 303Guy's picture, I suspect that the effect of the cream of wheat was to effectively peen the base of the bullet out to the edge, where the peening pressure was least due to the vortex turning and reversing towards the breech along the bore, and the bullet metal/paper peened together at the trailing edge.

If this bullet base cupping is due to the mixed phase flow (and not bullet base gas pressure) it should be more observable in cream of wheat type loads, and high case capacity loads of slower burning powders. It should be not be observed in loads using very high combustion rate powders, irrespective of peak or high average pressures. Examination of the bullet base, particularly with softer bullets, may also indicate a correlation between the roughness of the bullet base and cupping of the base due to impingement of the solid phase of the flow (this doesn't necessarily have to be the case as it depends on the relative hardness of the solid phase and the bullet base, and the velocity of impingement).

This also gets me to wondering about the folks who have reported shooting unlubed bullets with a cream of wheat filler. If the above ring vortex theory is true, I suspect that you might be able to do the same thing with a slow burning charge of powder. Even though the powder particles were burning (and thus base temperature/gases were just as hot), the influence of the mixed-phase flow vortex at the base of the bullet would reduce the gas pressure and leakage at the bullet/bore junction and reduce gas cutting. You could experiment with this by either using a large load of slower burning powder, or a duplex load of relatively slow burning over faster powder. In the latter case the idea would be that as long as the slow burning powder were ignited and burning, the gas temperature at the base of the bullet should be close to the adiabatic flame temperature of the propellant, demonstrating that the reduction in leading was due to the reduction in pressure at the bore/base edge and not related to 1) any reduction in gas temperature by the "filler", or 2) any mechanical plugging of leak paths by the solid particles.

Best regards,
DrB

303Guy
08-02-2011, 04:34 PM
This also gets me to wondering about the folks who have reported shooting unlubed bullets with a cream of wheat filler. If this theory is true, I suspect that you might be able to do the same thing with a slow burning charge of powder.
I have found that boolit bases are peened and cupped with high loadings of slower powders just like DrB has predicted!

I'll need to check but I seem to think that with higher load densities and only a small amount of wheat bran, the base peening and cupping is less than without. It is hard to assess due to the damage sustained by the boolit on impact in soft sand. I'd have to use damp rags as a catch medium to see for sure.

Rebate base showing cupping.
http://i388.photobucket.com/albums/oo327/303Guy/BaseCupping.jpg

Strong cupping with pronounced edge rim.
http://i388.photobucket.com/albums/oo327/303Guy/214grIXTAILLESSFIRED.jpg

All my photo's seem to support DrB's hypothesis.

DrB
08-03-2011, 04:29 AM
Thanks for sharing, 303guy. Always great to get a look at reality!!!

Btw -- you would think the peening would show a variation in impact marks (if individually discernible) from the centerline radially out. I would expect more normal/rounded impacts in the centerline, and more oblique impacts and/or streaking or nothing as you progressed outward toward the bore.

In a couple of your pictures it almost looks like there might be a distinct central circular area that's been peened more/more normally? Can you discern any radial variation in the peening?

Lastly, I'm curious about the paper patching not separating on the peened loads... was this consistent or perhaps a shot or two fluke? If consistent, do you think the peening had an effect on the failure of the patch to peel (I don't see why it should -- seems like the patch should still separate fine except maybe at the trailing edge)?

Best regards,
DrB

303Guy
08-03-2011, 06:02 AM
The peening appeared consistant. All the loads with wheat bran filler seem to retain at least the rear section of the patch at lower pressure levels and the base ring at higher pressure.

http://i388.photobucket.com/albums/oo327/303Guy/BSAMPATCH42GR.jpghttp://i388.photobucket.com/albums/oo327/303Guy/BSAMpatchfragment.jpg

These came from boolits like this one.

http://i388.photobucket.com/albums/oo327/303Guy/MVC-066F-1.jpg

Doc Highwall
08-03-2011, 09:07 AM
I am not sure that it is peening from the powder as much as I would believe that the powder was compressed against the base of the bullet upon ignition.

The powder when first ignited starts burning at the back of the case near the primer, pushing a wad of powder against the base of the bullet leaving a imprint on the base of the lead bullet, before all of it is ignited within an inch or less of travel down the bore.

To say that the powder peened the base of the bullet it would have to be traveling faster then the bullet like a sand blaster.

The powder and gasses cannot travel faster then the bullet while it is in the bore to peen the base of the bullet, but when the bullet leaves the muzzle the powder gasses can travel faster but only for a short distance.

felix
08-03-2011, 10:09 AM
Primers fire at greater than the speed of sound. ... felix

DrB
08-03-2011, 11:06 AM
I am not sure that it is peening from the powder as much as I would believe that the powder was compressed against the base of the bullet upon ignition.

The powder when first ignited starts burning at the back of the case near the primer, pushing a wad of powder against the base of the bullet leaving a imprint on the base of the lead bullet, before all of it is ignited within an inch or less of travel down the bore.

To say that the powder peened the base of the bullet it would have to be traveling faster then the bullet like a sand blaster.

The powder and gasses cannot travel faster then the bullet while it is in the bore to peen the base of the bullet, but when the bullet leaves the muzzle the powder gasses can travel faster but only for a short distance.

I'm not sure that the velocity of the solids would be great enough to leave an imprint from impact, either, vs just leaving an imprint from acceleration.

However, I do think the solids can travel faster than the bullet up until they reach the bullet base and turn outwards towards the bore. I think they'll turn outwards because those particles impacting the bore surface will loose velocity and travel rearward relative to the bullet base.

303Guy
08-03-2011, 04:44 PM
Either explanation could do it I should think. Considering that My boolits have a fibrous wad under them it would seem feasable that the centre of the wad is being forced up against the boolit base with more pressure in the centre being transmitted to the boolit. Powder might behave in the same way while still burning at launch.

The wad formed by wheat bran whould have drag on the sides which might explain the higher force transmission at the centre.

There has got to be a degree of gas impingement onto the boolit base at a molecular level, in the bore as that is how gas pressure is transmitted but whether the burning powder granules are carried with a greater velocity than the boolit itself would be speculation on my part.

I had actually thought the base cupping was caused by the boolit being swaged down with gas pressure acting on the base causing the sides to drag rearward. The sample without filler seems to indicate otherwise.

So, I'm open to suggestion here.

Oh, I have samples of gas checked boolits that also show base cupping. Worse still is that the base edges were uneven after firing!

Here's a boolit with edge 'cupping'!

http://i388.photobucket.com/albums/oo327/303Guy/MVC-875F.jpg

It's the patch tail impression.

http://i388.photobucket.com/albums/oo327/303Guy/PIGGUN011.jpg

felix
08-03-2011, 04:58 PM
Next time you let water out of the tub, notice the circling motion. Same with boolits. The cavity appears AFTER the boolit starts moving. ... felix

303Guy
08-03-2011, 05:15 PM
Accepted but does it form in the thoat as the base is being sized or further down the bore when the boolit has had time to 'flow' as lead does? Or perhaps a little of both? And is it caused by peaning or simply unbalanced pressure?

felix
08-03-2011, 05:39 PM
Unbalanced pressure, really. Think tornado, typhoon, etc. ... felix

DrB
08-04-2011, 03:59 AM
RE gas impingement, there is nothing that necessarily requires there to be the same gas pressure at the base of the bullet and back in the bore gas behind a slug of solid particles (or vortex of solid particles). But reading again, I don't think that is what you were saying. The solid particles will, along with gas particles, transmit driving force to the bullet base (obviously they are transferring some momentum to it based on the impressions left from the force of their impact!)..

RE vortex, I'm missing something here? The pressure in the center of a vortex is reduced, not higher... so if a vortex spinning rifling wise about the axis of the bore centerline were having a significant impact, it would cause the bullet base to experience more pressure at the edges than the center (reverse cupping!). Am I misunderstanding the vortex axis you are proposing, felix?

Also, rifling doesn't constitute much in the way of vanes to impart angular momentum to the flow.... angular momentum is conserved just like energy, mass, etc, so angular momentum has to come from somewhere.

Seems to me there really has to be a ring vortex in the bore behind the bullet because of the boundary conditions, particularly when you get farther down the bore and bullet velocity has picked up. With a high fraction of solid particles in the flow, intuitively I think it could cause a significant lateral gradient in the pressure on the bullet base. The particles may compress and slug up or they may fluidize... I'm not sure if they slug up though it would have the same effect RE base cupping?

Man, more great pictures from 303Guy. That's really neat to see the paper patch shape swaged to the bullet base. What're the concentric rings on the one on the left? Is that unfired vs. fired, and the bullet is from a reamed cavity mould (the concentric rings are tool marks in the nose of the cavity)? :? Also, I wonder if you'd get any cupping at all in that particular example if you trimmed the tail end of the paper patch? It looks like you can see the impression left from creases in the PP on the boolit base... seems to me the circumferential crease in the paper at the base corner could also result in extrusion of lead into the crease from the paper being pressed into the lead? Conversely, I wonder with a paper patch like that that goes over the base if you couldn't get cupping irrespective of much solids in the flow due to the same effect (the creasing of the PP at the circumferential edge)?

303Guy
08-04-2011, 06:42 AM
Its a nose pour mold and the rings are the base plug that I didn't machine very smoothly. It's an unfired and fired example.

Both of these were the same bar the filler.

http://i388.photobucket.com/albums/oo327/303Guy/DSCF5572_edited-1.jpg

I'm wondering how much effect the wad has in forming the cupping. The wad would tend to centre the pressure due to the drag on the edges. Both tese were low pressure loads.

I have had base distortion with a flat based boolit and gotten rid of it using a rebated boolit but I put that down to swaging in the throat of the two-groove with 80% bore to groove area.

http://i388.photobucket.com/albums/oo327/303Guy/MVC-108F.jpg

I'll try with setting the patch just a fraction forward of the base and see what happens.

357 Voodoo
08-04-2011, 02:11 PM
RE vortex, I'm missing something here? The pressure in the center of a vortex is reduced, not higher... so if a vortex spinning rifling wise about the axis of the bore centerline were having a significant impact, it would cause the bullet base to experience more pressure at the edges than the center (reverse cupping!).


DrB

Bernoulli's principle May have some effect in in this conundrum and if this were the case than with the lower pressure in the center the velocity of the gas and any particulate matter would be increased.

303Guy
08-04-2011, 05:23 PM
That's the lateral pressure. This a closed system with an expanding combustion chamber and burning matter all the way down the length of the bore.

DrB
08-04-2011, 06:03 PM
357 voodoo... Yeah, I'm just not seeing the bore coaxial vortex as a big thing, though? Pressure in the center should be lower with such a vortex. Also, I don't see anything that would create a strong coaxial vortex in the first place.

I can see a ring vortex with a mixed phase flow having a significant effect on bullet base pressure distribution, but I'm a bit out of my comfort zone with mixed phase flows as I never specifically studied them.

DrB
08-04-2011, 06:14 PM
Here's an example derivation of velocity distribution in a simple vortex.

http://www.desktop.aero/appliedaero/potential/vortexv.html

You can see there is a (mathematical, not physical :)) singularity approached as r goes to zero, and that pressure is decreasing with radius.... (this is why sometimes you can see vortex cores in humid conditions. The air cools enough from adiabatic expansion that water vapor condenses out).

A ring vortex is what you see in a mushroom cloud. Gases rush in at the bottom to the middle, turn upwards, curl over at the top, and rotate thusly. The center of rotation takes the form of a closed ring. In the mushroom cloud analogy the cloud head vortex part would be pushing up against the base of the bullet. The backwards rushing bore walls (in a bullet-fixed frame of reference) would tend to strip particles breachwards away from the bullet base, promoting the vortex circulation and decreasing pressure at the periphery of the bore as compared to the centerline.

303Guy
08-05-2011, 05:51 AM
Have a look at Jeff's photo. It shows dragging at the rifling impression. This type of dragging could conceivably contribute to base cupping.

http://img.photobucket.com/albums/v505/JeffinNZ/Shooting%20stuff/DSCN2684.jpg

DrB
08-05-2011, 04:28 PM
Is it dragging or swaging though? If you engraved the same bullet by hand (maybe would have to distribute the force over the whole base) would it show the same marks?

I think it might show those tabs independent of any gas pressure at all... and that would indicate it wasn't drag pulling the bullet base back but the base edge getting squished out by the rifling?

303Guy
08-05-2011, 08:12 PM
I think it might show those tabs independent of any gas pressure at all... and that would indicate it wasn't drag pulling the bullet base back but the base edge getting squished out by the rifling?Agreed. I used the term 'dragging' because that's where the displaced material went and I would expect there was dragging friction involed too. I thought it might shed some light on the processes involved. I suspect there several factors at play and that which factor (or combination of factors) dominate may vary.

The fact that a fibrous filler ripped Jeff's case mouths off indicate to me that there is a great deal of lateral friction involved.

DrB
08-06-2011, 03:20 AM
I didn't see Jeff's original post... hm. Different materials can matter a lot RE friction, but I see your point. Does the lube groove tell us anything? Can't tell if it's narrower just because of the riflilng cutting into the narrower depth of the groove or if there is reverse "dragging" of material forward.

I'm going to have to start recovering some bullets, 303Guy. This is fascinating, but looking at pictures (even these really nice closeups) isn't the same as holding 'em. Part of that's conceptual, I think, but a lot of it is perceptual... I forget what the numbers are, but a greybeard tool & die maker once told me the relative tolerances a person could see vs. feel... you can feel the difference between a very very small dimension, much smaller than you can see with a Mk 1 eyeball.

What're you using for your test tube?

I was thinking of trying to recover some of the NOE225107 37 grainers I've been shooting at 3200 fps using a 4'x8' by 4" thick piece of sandwiched polystyrene foam, backed by a gel bucket. At 3200 fps I'm expecting anything recovered would be too mangled unless I slowed it down with 8' of foam first... so hence the foam. I'm thinking sandwiching it with plywood will probably keep it ricocheted back into the middle of the foam if it starts to fish-hook out (although such a short bullet with such a wide meplat should be fairly unlikely to do that, at least until it's already going really slow). Seems to me the foam approach should work for recovering the bullet with less damage.

Best regards,
DrB

303Guy
08-06-2011, 03:47 AM
http://i388.photobucket.com/albums/oo327/303Guy/MVC-558F_edited-2.jpg

It's a segmented high tensile steel tube with an inner steel tube filed with sand. It has a 4140 steel disc at the bottom over and and a 4140 steel sand pot over that - not like any boolit will make it that far! Actually, I used to use water soaked rags as a catch medium to test for boolit performance. Then the boolits did make it into the sand pot.


Like you say, it can be hard to evaluate a recovered boolit due to impact damage.

http://i388.photobucket.com/albums/oo327/303Guy/20gr2205_146PP.jpg

DrB
08-07-2011, 11:59 PM
Yeah, but it could be abstract art. :) Got a name for that one?

303Guy
08-08-2011, 08:18 AM
I'd call it "Boolit Splash".8-)

How it stays together is amazing!

TRX
10-04-2011, 06:27 AM
The reason for a little cylinder play in a revolver and a boolit tough enough to pull the cylinder into alignment.

[facepalm] I always wondered if there was some reason the Nagant revolvers had the cylinder slide up and spigot on the back of the barrel. "Gas seal" never made any sense; the 7.62 Nagant cartridge wasn't very powerful, but it was unique to that gun. If they had wanted more velocity, they could simply have loaded it differently.

Though the Nagant cylinder arrangement was more complex than a Colt-style cylinder, the parts were all large and easy to make and fit, unlike the fussy Colt latch mechanism.

The Nagants also did amazingly well in competition, back in the day. Hmm...

Willbird
10-08-2011, 11:26 PM
Time + Money + an obnoxious attitude for an objective = accurate results. Most folks give up too soon for one reason or another, either good or bad. My experience has been not obtaining external ideas to get over an unexpected hurdle. The real excuse for many of these projects has been no profit potential for the results.

You are correct in using a "photo" apparatus for measuring the rotation. The boolit would have to be painted with something "hot" that can be read with the existing particle measuring devices. Finding an appropriately short half-life paint would be prohibitive???? Who wants a forever contaminated barrel? ... felix

Dr. Harold Vaughn (In the book Rifle Accuracy Facts) measured bullet rotation by placing a tiny disk shaped magnet in the bullet, in the nose of the bulled like a dig carries a frisbee in his mouth:-)...then fired the bullet through a wound copper coil, and measured the output of the coil.

That book is worth a read and a re read, I need to get another copy.

I think that if the rifling engraving is intact, then we can calculate bullet rpm just like we can say a 1/4-20 threaded bolt must rotate 10 turns to move 1/2" out of a threaded hole.

Bill

303Guy
10-11-2011, 04:42 AM
On trailing edge failure, I posted this on my thread;

http://i388.photobucket.com/albums/oo327/303Guy/001-1.jpg
This being powder kernel peening.

I tried a wad under the patch skirt and had this result. (The one on the right).

http://i388.photobucket.com/albums/oo327/303Guy/40gr_192gr_nofiller.jpg

Then gearnasher tried it out on the range and had rather interesting results;


... this morning I loaded some of my .45 Colt NEF loads, but this time added some .025" heavy, waxed card stock from a beverage container between the powder and patched boolit base. This cut my 100-yard group size literally in half, with no other discernable effects.

DrB
10-18-2011, 03:33 AM
That's an awesome experiment. Thanks 303guy.

1bluehorse
10-20-2011, 04:28 PM
Bret, the good Doc is figuring the size of the retro rocket required to compensate for any directional errors during launch. He is finding out compensation is too minuscule to be cost effective. His disclaimer deals with the obscurity of the actual real-time summation of the erroneous vectors (direction and speed) to work with. His discourse is just thinking out loud. ... felix

Uhh, yeh, okay, thanks...:holysheep

303Guy
10-28-2011, 08:05 PM
In the quest to minimize trailing edge damage I tried ball powder as has been suggested to be more cast boolit friendly. Well, preliminary results confirm that.

http://i388.photobucket.com/albums/oo327/303Guy/W748205PPtrial001.jpg
Ball powder - W748

As opposed to stick powder - Varget/AR2208
http://i388.photobucket.com/albums/oo327/303Guy/30grFILLER_35grNOFILLER_2209_208gr-1.jpg

DrB
10-29-2011, 02:08 AM
The peening or whatever it is on the varget bullet really goes all the way to the edge. What were the loads?

Is there a theory as to why? Single vs double base? I'm also kindof surprised the peening is so heavy all the way to edge, and so coarse. It would be interesting to see a few grains of the powder besides the damaged base of the bullet... Do they roughly match the craters in size?

303Guy
10-29-2011, 06:00 AM
The craters do match the powder granules. The charge was 36gr Varget on that worst one. I no longer have that sample but here is a pic of another boolit with the granules placed on top to give some idea. The reason for the impressions actually cutting the boolit edge turns out to be that it was seated just below the neck (an error on my part).

http://i388.photobucket.com/albums/oo327/303Guy/BoolitBasePeening003.jpg

I suspect the shape and hardness of the granules plays a role. Ball powder may have a slower initial burn hence a lower impact or shock against the soft lead. It is likely that ball powder has a self cushioning property due to its shape. I can't help thinking it's the self cushioning effect. Maybe not on its own but partially at least.

TXGunNut
10-30-2011, 01:09 PM
Great thread, need to set aside a few weeks to read it along with the reference materials to begin to understand it. My background is sales and marketing, not physics.
I'd like to defend or maybe explain why the gun rags don't print the tech articles that would be helpful or interesting to some of us. Mike V has written about it, I've heard other gun writers talk about it as well. Magazines are owned by corporations that must produce a profit for the shareholders. A large part of the revenue is from advertising. Folks who purchase ads want to reach as many people as possible and they like it when their products are mentioned favorably in the articles. It stands to reason that magazine editors want articles that will interest the widest spectrum of readers. They think those readers want to see the latest products (helps with ads, too) and aren't much interested in the science behind it all. Why put resources into technical articles when the target market probably can't balance a checkbook?
I agree that a magazine such as Handloader should find room for articles on this and other technical subjects but the editors apparently disagree. Could be why the only gun magazine I receive is American Rifleman. I used to subscribe to others but lost interest when the ad pages outnumbered the article pages and the articles often heaped high praise on a product advertised nearby.

airamb
11-09-2011, 03:39 AM
I very much agree to Micheal Petrov, I've read the book though.

303Guy
11-11-2011, 01:24 AM
I tried a load of 34.8gr W748 under a 194gr patched with printer paper boolit (which was smaller than previous) and the base survived essentially intact. I repeated the test but filling the case with wheat bran. That left a tail ring of the patch on the boolit and base edge flattening with some edge feathering. There was of course an increase in pressure and boolit damage from impact.

The relatively undamaged boolit base - there is peening.
http://i388.photobucket.com/albums/oo327/303Guy/194grwithwithoutWB004.jpg

FirstBrit
11-13-2011, 06:24 PM
I'd REALLY like to see some checked and pb examples of base cupping. Reality is, and I've never held one of these in my hand and seen what it looks like.

Best regards,
DrB
EDIT 7/12/2011: No one has a load they can suggest that they think will produce cupped base bullets with and without gaschecks? Or just not in .357 Magnum?

Hello DrB

In response to your request I did some testing about 7-8 years back on my S&W Mod. 27 with 180 gr. PB cast bullets from a Magma mould. Basically these tests were to give me some insights into the problems of gas cutting with plain based lead bullets and the influences of alloy hardness and powder burning rate.

Basically 6 loads were tested with 3 alloy strengths BHN 10; 16 and 22.
Series 1 control 5,0 gr. VV 3N37 with 4,4 gr. buffer similar to Buffer Mix # 47 ( Int. Ballistic Products Inc.)
Series 2 control 10,5 gr. VV N-110 with 2,2 gr. buffer.
Series 3 test 4,2 gr. VV N-320
Series 4 test 6,5 gr. VV N-320
Series 5 test 8,0 gr. VV 3N37
Series 6 test 13,0 gr. VV N-110
Hardness of the bullets in the photos is BHN 10, 16 and 22 from left to right.
The idea for the buffered loads was to completely eliminate any gas cutting which did work well. Gas cutting was examined visually and measured in terms of base and front band diameters on the recovered bullets.
My understanding is the maximum strain/stress for these bullets is incurred at P-max. and bullet travel at P-max varied from about 0,175" to 0,540" as predicted by Quick Load software program. Similarly P-max. varied between 19-36k psi.

Based on these limited results I only observed cutting with the buffered loads which as one might expect was more pronounced with the softer alloy. Clear indentation of powder granules was seen in series 6 and nearly all others showed signs of "pimples" or peening with the exception of the harder alloy in both control series.

Some other general observations. Gas cutting was more prevalent with the softer alloy and in the series 4 was so pronounced one can hardly see the groove markings which correlates with rear band diameter of 0,330-0,348".

I am sure these tests are no way conclusive but maybe it will work as a new piece to the overall jisaw puzzle.

Best regards,

Adrian, Germany.

http://castboolits.gunloads.com/imagehosting/thum_72184ec042ad174aa.jpg (http://castboolits.gunloads.com/vbimghost.php?do=displayimg&imgid=2684)http://castboolits.gunloads.com/imagehosting/thum_72184ec042baadf44.jpg (http://castboolits.gunloads.com/vbimghost.php?do=displayimg&imgid=2685)http://castboolits.gunloads.com/imagehosting/thum_72184ec042c986aaa.jpg (http://castboolits.gunloads.com/vbimghost.php?do=displayimg&imgid=2686)http://castboolits.gunloads.com/imagehosting/thum_72184ec042d98f76b.jpg (http://castboolits.gunloads.com/vbimghost.php?do=displayimg&imgid=2687)http://castboolits.gunloads.com/imagehosting/thum_72184ec042e78ccce.jpg (http://castboolits.gunloads.com/vbimghost.php?do=displayimg&imgid=2688)http://castboolits.gunloads.com/imagehosting/thum_72184ec042f5e5b15.jpg (http://castboolits.gunloads.com/vbimghost.php?do=displayimg&imgid=2689)

303Guy
11-14-2011, 12:31 AM
Thank you for the informative post, FirstBrit. Good to have you with us. :drinks:

geargnasher
11-14-2011, 12:47 AM
DrB, I happened to have a handfull of examples laying on the desk here, just about every single boolit I've recovered that was weaker than the peak PSI of the load cupped it. I don't have any examples of cupped gas check boolits handy, but I've seen it plenty of times there too with hot loads.

http://castboolits.gunloads.com/imagehosting/thum_89094ec09d2ee72fc.jpg (http://castboolits.gunloads.com/vbimghost.php?do=displayimg&imgid=2693)

Gear

303Guy
11-18-2011, 08:03 PM
I do believe I have the solution for my trailing edge damage problem. I increased the powder charge to completely fill the case (AR2209/H4350) and place a card wad under the boolit. I also used a slightly smaller boolit with thicker paper (printer paper).

http://i388.photobucket.com/albums/oo327/303Guy/44gr2209194grPPCARDWAD006.jpghttp://i388.photobucket.com/albums/oo327/303Guy/44gr2209194grPPCARDWAD004.jpg

One can see by the primer that pressure was moderate. I dare say muzzle blast is going to be notice-able with that short barrel even with the suppressor!

I've had the suspicion that it is the powder slamming into the boolit base that has been causing the base cupping, peening and edge feathering. That's what motivated me to take out the air-space altogether. It seems to work, with stick powder anyway.

P.S. For those who don't paper patch, the paper patching allows for full j-word velocities (if not higher) at lower pressure and often with better accuracy - if it's done right.

popper
02-22-2012, 02:04 PM
Anybody ever test sizing base first to clean up the edges on a PB CB? Tags from rifling should be the same. Any evidence that dacron filler will reduce base powder peening?

303Guy
02-22-2012, 04:01 PM
Any evidence that dacron filler will reduce base powder peening?Well, it seems like it might even. I got hold of some some filtration medium and it's heavier stuff and takes more to fill the case. Comparing primer flattening between H4227 loads and WW748 loads, there was no boolit base damage or peening at all.

http://i388.photobucket.com/albums/oo327/303Guy/22gr2205214IMP006-1.jpg

(To compare primers I zoom a photo up on the screen so I can take an accurate measurement then calculate the % flattening. I've done a thread on this).

popper
02-23-2012, 02:40 PM
Thanks. I've notices a marked increase in accuracy between H4895(short stick) and Lever powder( small ball) in 30-30, wondered if peening is the problem. Ran some RD 311 170 PB butt first through the sizer. I'll shoot them and see if there is a difference. Bases have a sharp edge, not frilly like running them nose first. I shoot at a commercial range and have no way of recovering bullets.

craig61a
10-05-2012, 05:07 PM
Have to get around to it someday...

JIMinPHX
01-24-2013, 12:51 AM
Stupid question....

How does the "pressure" differ between a fast or slow powder? If the velocity is the same, how would this affect flight?

Reason I ask, the same bullet can fly extremely well whether "pushed" by Bullseye or Unique. Help me understand the logic...

Thanks,

Chris in MO

I did some experiments a few years ago with a .44 mag & found that a fast powder would deform the boolit more than a slow powder. I fired two loads with the same muzzle velocity into crumb rubber & recovered the boolits. The ones that were pushed by H110 were hardly deformed at all. The ones pushed by Unique were deformed noticeably. I don't know if I still have the pictures or not.

Edit:
This may be of interest - http://castboolits.gunloads.com/showthread.php?81258-BNH-Vs-Speed
Post #54 on page 3 shows the different deformation with different powders.

popper
01-26-2013, 05:16 PM
JIm - after reading the delirious rants of all 5 pages of that thread BNH-Vs-Speed, I conclude that your original alloy was too weak (toughness, not hardness) for the pressure, the CBs were too small as evidenced by the lead coating on the GCs and that powder kernels at HV do peen the BC base. Note also that at the same pressure, the force on the base of a larger cal is greater, thus possibly greater base damage. The pics of recovered CBs is of value to those of use with limited casting and shooting experience.

JIMinPHX
01-26-2013, 11:47 PM
Popper, please help me understand a few things.

1) why do you say that the toughness of the alloy is the problem & not the hardness? Also, how would you make the alloy tougher?

2) Why do you say that lead on the GC means that the boolits were undersized? That doesn't make intuitive sense to me. I expect that I am missing something.

I agree about powder kernels peening the boolit bases. On another thread somewhere, I had posted pictures of boolit bases from 150-gr SWCs that had been fired from a .357 mag with different powders. Some of the bases looked pretty clean, while others were very pock marked. I don't recall which were which.

JIMinPHX
01-27-2013, 12:05 AM
I couldn't find the original thread, but I did manage to find the boolit base picture in my forum archives. If anyone knows how to search for a picture in a thread by the name of the picture, please let me know.

Thanks,
Jim

http://castboolits.gunloads.com/asset.php?fid=10536&uid=3614&d=1251127612

popper
01-27-2013, 03:33 PM
Hardness (BHN) and toughness -not breaking apart are related but not exactly the same. Your first pics showed rifling skidding, as you changed the alloy(more WW) it got better. You could also add sulfur or copper for strength but not the brittleness of more antimony. Your pic also had lead smeared over the GC. If the GC & CB are sized properly, there is no space between the GC wall and the bbl for lead to collect. 303guy's pics of deformed bases was informative to me. I was surprised to see pics of unique also damaging the base, as it's small and soft. Nonburning powder gets thrown against the base at high speed and I can see the hard stick stuff peening. My post was kind of a gripe about 5 pages that were pretty much argument and didn't really answer much or provide info.

detox
02-23-2013, 02:22 PM
Sorry, but a read very little of this tread, I just thought a would ask this question. Do fiberwads protect against base deformation and cushion bullet when fired. Seems these are the best thing to use with flat based bullets. I want to try some in my .357

303Guy
02-23-2013, 11:12 PM
I've found polyester to protect boolit bases. I used about double the recommended amount but that was over a fairly full case of powder.

Wolftracker
05-24-2013, 02:34 PM
I have been experimenting by using my swage dies to alter cast bullets. Specifically, I've been changing nose shape and making the bases hollow (hollowpoint punch) and a semi-boattail shape by reversing the bullet in my 3/4 E point former so the trailing edge isn't really contacting the lands much, only a bearing surface between the base and the roundnose point contact the lands. This bearing surface is .4525 in diameter. They shoot very well with minimal leading in testing, so far. They also are a joy to reload. Recovered bullets show no damage to the bases. My groups are 31/2 to 4 " at 25 yards with a Taurus PT1911 in 45ACP, using a sandbag rest. I used 6.1 gr. of Auto Comp, CCI 300 primers and mixed brass. The trailing edge of your typical cast bullet may have less effect than we think. This design allows excellent allignment in the case, no size reduction from taper crimping and sufficient contact with the lands to provide accuracy.

303Guy
05-25-2013, 09:29 PM
This design allows excellent alignment in the case, no size reduction from taper crimping and sufficient contact with the lands to provide accuracy.Any chance of a photo? I'm thinking of reducing the base of my boolits to prevent the rifling lands from squeezing those little bumps into the base. In effect, a bore-ride boolit base, but very short and with a taper step down so that the step itself doesn't form irregularities from the rifling.

Rattus58
05-30-2013, 01:47 AM
So I had a thought... Probably not a new one, but I haven't yet come across it exactly. I'd appreciate the feedback of the more experienced/observant cast bullet shooters out there. "Papa smurf" had a related thought so I know others are thinking along the same general lines.

You would think since immediately prior to departure from the bore the base edge of a bullet is confining the gas pressure and restraining inertial wobble that a major impact to accuracy would be the beginning of blow out of this trailing edge due to gas pressure (+ other forces), and subsequent net off axis forces imparted to the bullet. Asymmetric venting during departure would impart forces, and further erosion/scarring could additionally influence the creation of net off-axis forces. Non uniform release of inertial forces could also tend to throw the shot.

Trailing edge failure could explain a lot of the correlation between bullet hardness (and gas checking) and better accuracy at higher velocities (and higher muzzle pressure). A stronger bullet trailing edge should result in smaller imperfections at departure, more uniform mechanical release, and thus less net off axis forces, and better accuracy.

It would be neat to examine bases of plain base lead bullets below and well above the velocity where accuracy had significantly degraded, if you could recover them without much impact damage and look for gas erosion/scarring/fracture of the base surface, and particularly imperfections of the base edge where it departs the muzzle.

Any thoughts, observations, references to prior discussion of same?

Anyone ever shot a uniformly soft lead bullet vs. one with just the very base quench hardened? Or how about a gas check that was just a crimped on ring (a cup with no bottom, but a perfectly flat bottom edge) extending just past the base of the lead bullet?

Best regards,
DrB

The bullet as it leaves the muzzle is affected by not only the base (use a wad or GC in my opinion) but also the muzzle itself. A nick on the crown can ruin a bullets path in my experience.. which until you learn of it may be forever frustrated till someone with a more trained eye discovers your potential... but it is an extremely easy fix. That's one... poor casting... especially with hollow bases... poor pours... all have caused me huge accuracy problems... gas checks and wads.... :grin: (in my opinion.) I'm using as pure a lead as i can... :grin:

Rattus58
05-30-2013, 01:49 AM
Sorry, but a read very little of this tread, I just thought a would ask this question. Do fiberwads protect against base deformation and cushion bullet when fired. Seems these are the best thing to use with flat based bullets. I want to try some in my .357I use walters wads with my heaviest loads or gas checks... walters wads are great as are some other homemade concoctions I've shot. These are in muzzleloaders with the softest lead I can find.

beefie
08-02-2013, 02:59 PM
I used to have rounded base problems. I tried running the mold colder than the alloy, having a thermometer in the lead pot,, using a bit more antimony, etc, and those tips helped. But what REALLY fixed the problem was using a spout-pour ladle, instead of the open dipper supplied by Hensley and Gibbs.

beefie
08-02-2013, 03:05 PM
It is a shame that Lee quit making their pistol rest. With good ammo, guys, a good 1911 groups 2" and less at 50 yds, for 5 shots. Not 3" at 25 yds. I had one wonderful variant, which was an Old NM slide group, on an Essex frame, back in 1970, which really would (from the Machine rest) group into 9" at 200 yds. This was with Remington jacketed swc match ammo, 185 grs at about 750 fps. When I looked thru the sights, they were aimed over 20 ft above the target! :-) A friend had to go downrange about 150 yds, to the side and behind a big tree, and call out my hits, until I got on target. I recall a mag article of a guy doing the same, IIRC, it was with hot jhp loads, in a 38/45 version of the 1911. the guy wanted to do well in 200 yd metallic silouette shooting, using a 1911, but the retained momentum was not high enough to knock over the rams.

BAGTIC
08-06-2013, 01:35 AM
One reason base defects are more deleterious that nose defects is that they are farther from the axis of the bullet. The farther an imbalance is from the axis of a rotating object the greater the imbalance.

popper
08-06-2013, 07:09 AM
Glue a string to the nose and pull it around. CB follows the string pretty good. Now push it around with a pencil point on the base. Not so good?

303Guy
08-06-2013, 05:33 PM
Good analogy. That applies to muzzle blast. I'm not sure how trailing edge damage effects a stable boolit in flight, after all, there is a shock wave propagating off the trailing edge so perhaps the trailing edge gets tugged in a spiral with the same pitch as the boolit spin. This might accentuate as the boolit travels down range rather than dampen as a muzzle blast tipping would. On the other hand, perhaps the muzzle blast tipping is made worse by the unequal trailing edge.

Consider also that some boolit designs are less stable than others. A flat nose boolit is less stable than a round nose of the same length (not weight) and a spire point is more stable still (unless it suffers nose slumping in the bore). A spire (or spitzer) point will likely dampen any yaw and related spiral flight path while a flat nose might increase the yaw and spiral boolit path (not sure on that!) This is of course speculation. I'm getting my information from; http://www.border-barrels.com/barrel_twist.htm. Check it out for yourselves and you can see how nose shape affects stability. It shows required spin but is a bit confusing as it looks like it's upside down. You'll see what I mean.

Now playing around with the twist calculator one will find that small changes to the boolit base (the boat tail) causes large changes to stability for small changes while changing the nose shape has a smaller effect (and surprisingly, some shapes are more stable in the transonic zone!? Bear in mind it's a software simulation - the algorithm may be inadequate in that zone).

Westerner
08-09-2013, 04:04 PM
This thread has been on for a long time. I shoot 45/70 in a Sharps action with 30-1 Lead/tin mix. It took a plastic card wad between a smooth card wad and perfect base of a 500 grain round nose lubed bullet to print less than 1.5" at 100 yards. The slug comes from a nose pour mold that is held closed for 3 minutes before it is dropped into water in a shallow pan with a cushion at the bottom. Each slug is put into a shallow reloading tray nose down between casts. Nothing touches the bottom of the bullet until it is pushed into a slightly belled case mouth on to the two wads and powder charge and taper crimped. Roll crimps scrape the bullet shaft so avoid that step.

geargnasher
10-05-2014, 10:09 PM
The formula for predicting torsional stress imparted to the engraved bullet by the rifling is as follows:

F/A = .00000697 · ((WrV²) / (Pldn))

Where

F=total force in pounds exerted upon the bullets by the leading sides of the lands.
A=total area in square inches of the leading sides of all the lands taken by the bullet.
F/A=force per square inch upon the leading sides of the lands.
W=weight of bullet in grains.
V assumed to be equal to the muzzle velocity of the bullet by which the above formula becomes empirical.
P=pitch of the rifling in inches.
n=number of grooves in the rifle bore.
r=radius of bullet in inches.
L=length of bullet which takes the grooves (bearing surface) in inches.
d=depth of rifling grooves in inches.

This is taken from F. W. Mann's book The Bullet's Flight.


Gear

popper
10-06-2014, 11:19 AM
F/A = .00000697 · ((WrV²) / (Pldn)) I did the calc for my 150 gr in the AR308. 4389 psi? Very close to my calc a few years ago, for the rifling engaging psi. Once the boolit is spinning, force drops by x10. Couple years ago I did some 'twist till break' tests on HT #2 30 cal rifle (RD mould). I could apply ~ 30# with 6" vise grips and get a breaking force of ~360#, ~4741 psi. The weak point is a the lube grooves (smaller dia), so this is a good formula. I posted a psi number (435 psi) for softening of HT Pb62%Sn, which is close to HT #2 alloy. Note that number is NOT for fracture, just the force applied to a disk used to measure the BHN (Hv - vickers hardness) of the alloy. The Hv at the disk center remained ~10, IIRC ~ 3 at the edge, indicating that the alloy softening occurs primarily at the outside of our boolits AND doesn't recover while moving down the barrel. We wonder why we get gas cutting? It also adds some validity to Larry G's twist vs accuracy discussion. Also adds validity for Cu jackets & to some extent, PCing.
A flat nose boolit is less stable than a round nose of the same length primarily because the FN slows down faster and enters the transonic region earlier (~mach 1.5-0.8).

Ricochet
10-06-2014, 11:32 AM
I'm presently reading Mann's "The Bullet's Flight...", only to page 127 I think so far. I found interesting his experiments with extremely short barrels so that the pressure at exit was higher than the obturating pressure of the lead, producing impressive bullet expansion from the rear. His brother named this the Putty Plug Principle, meaning that the bullet in the bore under pressure was like a plug of putty, confined by the bore walls to keep it cylindrical. He found the effects of base damage to be minimal, but he was always using wads. He also found that deliberately damaging the muzzle in quite severe, asymmetrical fashion shifted groups with relation to point of aim without affecting group size. I think a takeaway is that wads and fillers were widely used in the 19th century for good reason with lead bullets. We probably should do likewise.

geargnasher
10-06-2014, 12:22 PM
F/A = .00000697 · ((WrV²) / (Pldn)) I did the calc for my 150 gr in the AR308. 4389 psi? Very close to my calc a few years ago, for the rifling engaging psi. Once the boolit is spinning, force drops by x10. Couple years ago I did some 'twist till break' tests on HT #2 30 cal rifle (RD mould). I could apply ~ 30# with 6" vise grips and get a breaking force of ~360#, ~4741 psi. The weak point is a the lube grooves (smaller dia), so this is a good formula. I posted a psi number (435 psi) for softening of HT Pb62%Sn, which is close to HT #2 alloy. Note that number is NOT for fracture, just the force applied to a disk used to measure the BHN (Hv - vickers hardness) of the alloy. The Hv at the disk center remained ~10, IIRC ~ 3 at the edge, indicating that the alloy softening occurs primarily at the outside of our boolits AND doesn't recover while moving down the barrel. We wonder why we get gas cutting? It also adds some validity to Larry G's twist vs accuracy discussion. Also adds validity for Cu jackets & to some extent, PCing.
A flat nose boolit is less stable than a round nose of the same length primarily because the FN slows down faster and enters the transonic region earlier (~mach 1.5-0.8).

I maintain that twist vs. velocity is a matter of bullet balance. Past a certain RPM the imbalances imparted to the bullet through NORMAL casting and loading techniques really sling the bullets out of the group. Most of that imbalance is probably imparted during the first inch of the firing event. Not sure how or if any work-softening of the driving bands during the engraving event affects high-velocity accuracy, but I suppose it could contribute to obturation failure at the trailing edge of the lands if the alloy is softening.

The results of my calculations in several rifles and handguns is surprising to me, and interestingly I've found that with my limited data is a positive trend to groups opening up when the leading edge pressure per Mann et al exceeds the Lee formula for chamber pressure vs. Brinnell hardness number of the alloy. I'm thinking with a little more data we might be able to note a valid trend here, where chamber pressure is ignored but only the rifling stress pressure vs. bhn is taken into account. To put that statement in perspective, one rifle load of mine that is one grain above the system's accuracy threshold has an estimated peak chamber pressure of just under three times the maximum recommended by Lee, but only a single-digit percentage above what I'll call the "land stress maximum" for the alloy. Below the land stress maximum groups were very good. This of course doesn't take into account any work-softening of the alloy when fired.

I've been told by a couple of people who ought to know to ignore rifling stress, it's insignificant. After doing the math and comparing calculated land pressure to a few groups with various loads, I'm not so sure that calculating land pressure and taking that into consideration when alloying and loading is a waste of time, it may not be. Take all this FWIW.

Gear

Forgetful
10-06-2014, 01:22 PM
Any way to case harden lead so you have a softer core?

geargnasher
10-06-2014, 02:52 PM
Paper or copper jackets.

Gear

popper
10-06-2014, 08:23 PM
Gear - I was interested in the boolit entering rifling torque part of the equation, slower twist, lower torque on the boolit. Like trying to screw a wood screw into steel hole. They always twist off.
rifling stress pressure vs. bhn is taken into account Darn now I have to check my old Xcel calcs for rifling force on the boolit going down the barrel. I used some pressure data from Larry G. to run all the calcs, but I didn't look at that one closely.

303Guy
10-07-2014, 12:55 AM
Flat nose boolits are less stable than round nose because they undergo higher tipping forces due to their shape. As they slow down their stability increases because the tipping forces diminish. Tipping forces always increase with velocity increase but with spitzers that increase is a little less than the increase in spin stability to a point.

SniderBoomer
10-07-2014, 04:45 AM
I Paper-Jacket my 45-70 boolits a lot, this is the base of a recovery, the paper was cut neatly at the base edge, not folded over. The load used no filler, the lead used was roofing lead and tested to BHN 5.

Is the lack of filler the reason the powder indents are consistently on one half of the base like this?

To be honest, these are accurate as heck in my 1895, but if I can improve things some, why not.

http://i62.tinypic.com/2zhnnd2.jpg

Larry Gibson
10-07-2014, 04:05 PM
From post #163 above;

"I maintain that twist vs. velocity is amatter of bullet balance. Past a certain RPM the imbalances imparted to thebullet through NORMAL casting and loading techniques really sling the bullets out of the group."

and......

From; http://castboolits.gunloads.com/showthread.php?208186-RPM-Threshold-barrel-twist-velocity-chart


"The RPM threshold is that point where accuracy begins to deteriorate when the RPM is sufficient to act on imbalances in the bullet in flight to the extent the bullet begins a helical arc in flight or it’s flight path goes off on a tangent."

.............is there a difference between the two?

Larry Gibson

btroj
10-07-2014, 04:11 PM
Deleted

Bullshop
10-07-2014, 04:30 PM
From post #163 above;

"I maintain that twist vs. velocity is amatter of bullet balance. Past a certain RPM the imbalances imparted to thebullet through NORMAL casting and loading techniques really sling the bullets out of the group."

and......

From; http://castboolits.gunloads.com/showthread.php?208186-RPM-Threshold-barrel-twist-velocity-chart


"The RPM threshold is that point where accuracy begins to deteriorate when the RPM is sufficient to act on imbalances in the bullet in flight to the extent the bullet begins a helical arc in flight or it’s flight path goes off on a tangent."

.............is there a difference between the two?

Larry Gibson
Larry
I brought this up in another thread about high velocity with cast boolits but no one wanted to touch it.
If as you say the twist vs velocity is a matter of boolit balance then I have to wonder why I can get good accuracy with a 55gn .224" boolit fired in a 1/10" twist at over 4000 fps when used in a sabot but cant get good accuracy in the same twist barrel and boolit when fired in a .224 cal barrel.
It seems that if balance was the factor causing the difference why is one balanced and one not? What is it that is causing the imbalance in the boolit fired from the right caliber barrel and not in the one fired from an over caliber barrel in a sabot?
The torque of the boolit being screwed into the barrel has to be causing the imbalance if imbalance is the cause of inaccuracy.
It cant be RPM alone because shooting the same boolits at extreme rpm using a sabot is accurate.

geargnasher
10-07-2014, 09:11 PM
Bullshop, the sabot works the same way as a paper or copper jacket: By protecting the core from damage due to crooked start or, if it is even a cause, the rifling. Balanced bullets tend to shoot very straight, very fast, and are pretty much immune to RPM.

Gear

Larry Gibson
10-07-2014, 10:45 PM
Bullshop

Your question has been discussed and answered in several other threads on the subject. Post #172 is an excellent synopsis of the answer.

Larry Gibson

Bullshop
10-08-2014, 05:04 PM
Thank you

leftiye
10-09-2014, 05:37 AM
Larry
I brought this up in another thread about high velocity with cast boolits but no one wanted to touch it.
If as you say the twist vs velocity is a matter of boolit balance then I have to wonder why I can get good accuracy with a 55gn .224" boolit fired in a 1/10" twist at over 4000 fps when used in a sabot but cant get good accuracy in the same twist barrel and boolit when fired in a .224 cal barrel.
It seems that if balance was the factor causing the difference why is one balanced and one not? What is it that is causing the imbalance in the boolit fired from the right caliber barrel and not in the one fired from an over caliber barrel in a sabot?
The torque of the boolit being screwed into the barrel has to be causing the imbalance if imbalance is the cause of inaccuracy.
It cant be RPM alone because shooting the same boolits at extreme rpm using a sabot is accurate.

I see where you are coming from. I agree that if there wasn't the factor of damage to the boolit that the .22 boolit shot in the .22 barrel would be the more accurate as the sabot would center and balance the boolit less well. Balance goes out the window along with straight starting, and correct launch with damaged boolits.

303Guy
10-17-2014, 04:50 AM
http://i388.photobucket.com/albums/oo327/303Guy/TRAILINGEDGEDISTORTION_zps872907e1.jpg (http://s388.photobucket.com/user/303Guy/media/TRAILINGEDGEDISTORTION_zps872907e1.jpg.html)

Here the trailing edge failure is quite evident. This is from boolit swaging.

s mac
10-17-2014, 09:19 AM
http://i388.photobucket.com/albums/oo327/303Guy/TRAILINGEDGEDISTORTION_zps872907e1.jpg (http://s388.photobucket.com/user/303Guy/media/TRAILINGEDGEDISTORTION_zps872907e1.jpg.html)

Here the trailing edge failure is quite evident. This is from boolit swaging.

By swaging do you mean an oversize boolit? Swaged in the bore?

Bullshop
10-17-2014, 10:56 AM
I see where you are coming from. I agree that if there wasn't the factor of damage to the boolit that the .22 boolit shot in the .22 barrel would be the more accurate as the sabot would center and balance the boolit less well. Balance goes out the window along with straight starting, and correct launch with damaged boolits.
Then breach seating or muzzle loading the boolit as was being done by target shooters at the beginning of the 20th century should completely eliminate any damage being done to the boolit upon acceleration and transition from case neck to being fully into the barrel.
Those same target shooters eventually went to fixed ammo as being equally as accurate as the former two methods of loading a breach loading target rifle if and when every precaution was addressed to achieve perfect alignment of boolit to bore.
Since the loads that were being used at that time were pretty much limited to black powder type pressures the torque being applied to the boolit from a state of rest to a spinning projectile was reasonably low compared to some of the pressures we load to now.
I submit that the torqueing of the boolit is a cause of damage to the boolit even when the alignment of boolit to bore is perfect.
Think of it like you were trying to turn a lead bolt with a steel wrench. The greater the torque the greater the chance the bolt will break. Sloppy chamber dimenti9ons would seem to fall into place with this idea as well as harder alloys for higher pressure, torque.
It also seems that longer boolits transitioning from case neck to fully inside the barrel may suffer internal structural damage from this torqueing at different interval for the length of the boolit. As the boolit is moving forward and beginning to take on the torque stresses at its nose the base end is not yet affected by the torque of the rifling trying to get the boolit spinning. I believe this can or may cause internal structural damage at a micro level. Greater pressure would give higher velocity which would cause greater torque pressure on that lead bolt example I mentioned.
Smack your steel wrench on the lead bolt with a big hammer and the bolt will break. Apply slow steady pressure within the strength limits of that bolts make up and it may just turn before it breaks.
Evidence of this uneven torque for the length of a boolit is evidenced by the uneven rifling engraving that can be seen on recovered boolits, especially revolver boolits. You will most often see that the engraving is wider at the lead drive band than at the base showing that there was some torque being applied to the nose before it was applied to the base.
A rifle chamber has some amount of unsupported free travel the boolit must transition to be fully inside the barrel just as does a revolver so is torqueing in the same manner. Is this not why we always see repeated that "FIT IS KING" because a tight fit would allow less damage to the boolit as the torque is being applied. The tight fit would help to contain the amount the boolit can deform just as does a breach seated boolit.
I can not deny that alignment is an absolute critical issue but I also submit that there is more to the equation than alignment alone. I also submit that high rpm alone is not the sole reason for imbalanced boolits shooting erratically but that the higher rpm (torque) is actually a cause for the damage causing the imbalance. Again this idea is evidenced by the fact that as load pressure is increased and so torque and rpm a harder alloy is needed.
Maybe I am off base and am sure someone will tell me so and exactly why but that is my reasoning and the way I see things now.

303Guy
10-17-2014, 02:09 PM
By swaging do you mean an oversize boolit? Swaged in the bore?Yes, swaging in the bore with a throat fitting boolit which is oversize for the bore.

popper
10-18-2014, 10:36 AM
Thanks Bullshop. I would love to do tests with different alloy using a multi-ton high speed pneumatic press to see what the failure mode of our boolits is. I do think the 'oversizing' of our boolits could be stidied also. I can agree with L.G.'s twist theory but I do think the kick-in-the-seat vs twist has a lot do do with accuracy. Now I'm wondering if PP actually helps launch.

Bullshop
10-18-2014, 09:19 PM
A slower longer push (slow powder burn) seems to be more forgiving than a fast kick in the but (fast powder burn) especially as peak pressures pass about 50% of max and go up from there even of peak pressures of both powder types are roughly equal. This too then would lend credence to the torque issue I was speaking of. Give the wrench a quick hard smack and the lead bolt might break. Apply pressure slowly and maybe it turns.

destrux
08-07-2015, 03:22 PM
I was getting a lot of trailing edge failures from air pockets in the base. Turned out to be my pot was pouring too fast into the mold, frothing the lead up. Slowed down the pour rate and held the mold closer to the nozzle and no more air bubbles.

natty bumpo
05-06-2017, 11:08 PM
So I had a thought... Probably not a new one, but I haven't yet come across it exactly. I'd appreciate the feedback of the more experienced/observant cast bullet shooters out there. "Papa smurf" had a related thought so I know others are thinking along the same general lines.

You would think since immediately prior to departure from the bore the base edge of a bullet is confining the gas pressure and restraining inertial wobble that a major impact to accuracy would be the beginning of blow out of this trailing edge due to gas pressure (+ other forces), and subsequent net off axis forces imparted to the bullet. Asymmetric venting during departure would impart forces, and further erosion/scarring could additionally influence the creation of net off-axis forces. Non uniform release of inertial forces could also tend to throw the shot.

Trailing edge failure could explain a lot of the correlation between bullet hardness (and gas checking) and better accuracy at higher velocities (and higher muzzle pressure). A stronger bullet trailing edge should result in smaller imperfections at departure, more uniform mechanical release, and thus less net off axis forces, and better accuracy.

It would be neat to examine bases of plain base lead bullets below and well above the velocity where accuracy had significantly degraded, if you could recover them without much impact damage and look for gas erosion/scarring/fracture of the base surface, and particularly imperfections of the base edge where it departs the muzzle.

Any thoughts, observations, references to prior discussion of same?

Anyone ever shot a uniformly soft lead bullet vs. one with just the very base quench hardened? Or how about a gas check that was just a crimped on ring (a cup with no bottom, but a perfectly flat bottom edge) extending just past the base of the lead bullet?

Best regards,
DrB

Very glad to have found this post as I have Just now begun doing just what you suggest which is examining recovered rounds. Not not claiming anything to be true but only what I have observered and surmised by these observations. The first observation is that the entire base of the bullets that I found is almost clean with little or no deformity save for a relatively small area that is craters from the burning powder. This area is almost always off center as it is a random event and does not encompase the entire surface of the base but only shows a small impact zone. It seems that the further from center that this cratered area is the more apt that the bullet is going to show signs of tumbling through the flight path as evidenced by the impact damage on the bullet. Some of these boolits show quite clearly that the boolit impacted on it's side Perpendicular to the normal axis of the intended direction.

caffe
05-28-2020, 07:10 PM
I would think that with 5" or less pistol barrels, you'll still have increasing velocity regardless of the powder used? If the stuff is actually consumed, how come there's still muzzle flash at night? All this depends upon the accuracy you require, of course. Me, as long as it 'll group 6" at 25yds, I'm ok with it. for several reasons. One, few "combat" matches require anything like that sort of accuracy and because nobody've ever proven that they can shoot that well when being shot-at, and defensive stuff is all I care-about. I dont have the needed time, money or reflexes to shoot the 100,000 rds per year needed to excell at top level matches these days and the matches have way too many stupid stages of fire that I'm not going to teach myself to do, cause they're stupid and it would cost me a lot of money that I'd rather put into long term storage food and bullion gold coins (especially with this virus thing being extant).

GooseGestapo
05-31-2020, 08:15 AM
The muzzle flash is from unconsumed oxidizer.
Gunpowder doesn’t require atmospheric oxygen to burn, as it releases it’s own oxygen.
When that unreacted oxygen hits the atmosphere, it ignites resulting in the flash, and inhanced “bang”.

popper
10-19-2021, 07:58 PM
unconsumed oxidizer.
Nope. Burning mode of smokeless powder is the primer heat turns the surface of the powder into a plasma that burns and creates pressure. The hot plasma exiting the muzzle heats the AIR to a high temp so it glows. Unconsumed powder with plasma coating will burn UNTIL the pressure is reduced and the plasma fails. The boom is the high pressure shockwave created at the muzzle. That shock wave has been accused of creating a SEE - it's pressure wave can reflect down the barrel. The exit fps of 'whatever/gas' is 2x the projectile fps.