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Thread: Cast bullet accuracy and trailing edge failure

  1. #1
    Boolit Master
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    Lightbulb Cast bullet accuracy and trailing edge failure

    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

  2. #2
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    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.
    When I think back on all the **** I learned in high school it's a wonder I can think at all ! And then my lack of education hasn't hurt me none I can read the writing on the wall.

  3. #3
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    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)
    Last edited by DrB; 06-30-2011 at 04:42 AM.

  4. #4
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    "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.

  5. #5
    Boolit Master
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    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.
    Evaluate everything you read for safety and use common sense.

  6. #6
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    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.

  7. #7
    Boolit Master
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    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

  8. #8
    Boolit Man
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    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.

  9. #9
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    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.

  10. #10
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    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.

  11. #11
    Boolit Master Char-Gar's Avatar
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    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.
    Disclaimer: The above is not holy writ. It is just my opinion based on my experience and knowledge. Your mileage may vary.

  12. #12
    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

  13. #13
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    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
    Last edited by W.R.Buchanan; 06-30-2011 at 01:33 PM.
    "It's not how well you do what you know how to do,,,It's how well you do what you DON'T know how to do!"
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  14. #14
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    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.

  15. #15
    Boolit Master Char-Gar's Avatar
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    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.
    Disclaimer: The above is not holy writ. It is just my opinion based on my experience and knowledge. Your mileage may vary.

  16. #16
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    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
    "It's not how well you do what you know how to do,,,It's how well you do what you DON'T know how to do!"
    www.buchananprecisionmachine.com

  17. #17
    Boolit Master Harter66's Avatar
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    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.
    In the time of darkest defeat,our victory may be nearest. Wm. McKinley.

    I was young and stupid then I'm older now. Me 1992

    About that rally point, the soap and mail boxes fall on blind eyes and deaf ears,the ballot box is manipulated,it would seem we are fast running out options....

  18. #18
    Boolit Master Harter66's Avatar
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    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.
    In the time of darkest defeat,our victory may be nearest. Wm. McKinley.

    I was young and stupid then I'm older now. Me 1992

    About that rally point, the soap and mail boxes fall on blind eyes and deaf ears,the ballot box is manipulated,it would seem we are fast running out options....

  19. #19
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    Quote Originally Posted by W.R.Buchanan View Post
    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!

  20. #20
    Everyone who shoots cast lead bullets should read Mann's book, at least once.

    http://books.google.com/books?id=QdQ...W.Mann&f=false

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Abbreviations used in Reloading

BP Bronze Point IMR Improved Military Rifle PTD Pointed
BR Bench Rest M Magnum RN Round Nose
BT Boat Tail PL Power-Lokt SP Soft Point
C Compressed Charge PR Primer SPCL Soft Point "Core-Lokt"
HP Hollow Point PSPCL Pointed Soft Point "Core Lokt" C.O.L. Cartridge Overall Length
PSP Pointed Soft Point Spz Spitzer Point SBT Spitzer Boat Tail
LRN Lead Round Nose LWC Lead Wad Cutter LSWC Lead Semi Wad Cutter
GC Gas Check