fit to throat confusion

[dahermit]

Rifle shooter wisdom gleaned from The Fouling Shot et. al. says: "...size the cast bullet to .0005 less than the diameter of the throat..." and, "...the bullet must fit the throat..."

After studying the diagrams of typical rifle chambers and reading many articles on the fitting of bullet to a particular rifle, I have observed the following.

All common (not custom) cast bullets have cylindrical driving bands. The "throat" of standard SAMII chambers are tapered.

Therefore, given a tapered throat (a 30-06 Ruger #1 in this instance), the actual measurement derived will be dependant upon where on the throat taper the measurement is taken. Up next to the chamber is .3135, further down towards the leade it is much less.

Just how then is a standard Saeco bullet with cylindrical driving bands fitted to a standard tapered throat by measuring? My practice up till now is to seat the bullet nose into the lands until it stops...I reason that the first driving band is then contacting the taper of the throat at some point along the taper. I have been sizing the bullets at .309, one thousanth over the .308 groove dia., been getting 100 yd. groups of slightly over an inch for 5 shots...most frequently 4 shots in under 3/4 inch, one shot usuall opending the group to 1 and 1/8 to 1 and 1/4 inch groups.

Regards,

dahermit

[MGySgt]

dahermit -

Your groups are outstanding but to answer your question, I need to ask a few questions.

What are the actual measurements of your rifle? - Chamber/Throat/Lead and bore diameter.

If you are looking at diagrams of chambers your are looking at the SAAMI standards and we all know that each rifle is a little bit different, chamber reamer new and sharp, or old and resharpened? Some chambers are a little bigger and some a little smaller.

For target work - yes the accepted is .0005 under throat/lead. If that is a hunting rifle, I would suggest you go .001 under throat/lead. (A little grit in the chamber can cause a bullet not to chamber or extract - been there, done that and didn't get the shot!)

If the throat/lead is tapered go .001 under the smallest diameter. The ones I have measured may have been tapered, but the difference may have been .0001 or .0002, not enough to worry about.

Drew

[Bass Ackward]

Rifle shooter wisdom gleaned from The Fouling Shot et. al. says: "...size the cast bullet to .0005 less than the diameter of the throat..." and, "...the bullet must fit the throat..."

After studying the diagrams of typical rifle chambers and reading many articles on the fitting of bullet to a particular rifle, I have observed the following.

All common (not custom) cast bullets have cylindrical driving bands. The "throat" of standard SAMII chambers are tapered.
dahermit


da,

Not sure what you are asking. The above quote from the Fouling shot is refering to a throat that has a ball seat type of throat. That is a machined space cut by the reamer with parallel sides that ends with a slight taper into the leade of the rifling. So it has an end taper will generally match the angle on the end of the rifling until it wears away. But the ball seat portion does not. Thus the above description.

So not all SAAMI throats are tapered. In fact, while case dimensions and headspace are standardized for safety, throat specs can differ wildly. This is why there is such a vast difference in what you will find from manufacturer to manufacturer.

Since the throat area of a reamer is the vanguard for the reamer, it cuts the farthest and takes the most wear. In order to make as many rifles as possible from a single reamer, these dimensions are made generous to accomodate wear and resharpening.

To molds, Lyman Loverin designed or style bullets are tapered. This is usually from a diameter that is slightly above bore (land height) to some larger dimension that is usually well above the bore (groove). For 30 caliber, my 311466 is .302 to .312.

Matching a taper is the hardest shape to fit, but often unnecessary as you have found out with your success. Loverins usually provide enough centering that they function well in standard mold offerings. If you can find one available. They seem to be favorites in the group buy section of this site. Bore rides are also supposed to provide centering and guidance to support enough of the bullet until the bands enter the bore and seal.

Did this answer your question?

[joeb33050]

You got your cartridge case neck. Further out you got your rifled barrel.
I've got plenty of cold swaged lead impressions of this area.
You got your throat, ball seat, leade, free bore, your knurlman, and I don't know what they are. Definitions vary, a lot.
I can draw what's in my guns, but don't know the names of these areas, and I don't think it matters. What does matter is the dimensions, and I thinkwe get screwed up on the names.
I've got a 30/30 bench gun that Mark Penrod built. The chamber:
At 2.080" from the base the chamber neck ends, and measures .326" dia.
There's a .3095"/.310" cylindrical section from 2.080" to 2.225" or .145" long.
Then the rifling starts, the lands may be tapered but I can't see it if they are.
I can successfully shoot .312" diameter 311/314299s in this gun, with the bands almost all in the case.
I can also successfully shoot .309" diameter 311/314299s in this gun, with the bands almost all the .145" out of the case neck.
I've won matches and shot groups in the high 2s with this gun and .312" diameter bullets, It averages a little under 1" for 5 five shot 100 yard groups.
I think that it shoots a bit better with the .309" bullets, but the difference is not great.
I think we would know a lot more if we got people to make impact castings of the "throat", got somebody to draw the picture, and then watched how different bullets/diameters shoot. If we had a library of these maybe we could learn something about this "throat"business.
Remember Harrison's warning about nose riding bullets and small lands, Maybe we should stop our love of the bore rider nose bullets and get some experience with groove diameter bullets.
I wish I could do the CAD drawings to keep records with, I think this would be worthwhile.
My gun/s shoot pretty good when the bullet does NOT fit the throat, whatever the throat is.

joe brennan

[Char-Gar]

I am anything but an expert and numbers and drawings give me a headache. In my simple mind, it is all around straightness. The bullet must enter the barrel straight to give it best accuracy.

The rifle chamber must be straight with the bore and the bolt head straight with the rear end of the chamber. The case needs to fit the chamber and not flop around. The fit of the bullet to the throat (or whatever) is just one more element in keeping things straight.

Throat (or whatever) designs and dimensions vary as do bullets and how they fit in the "whatever". When it is all boiled down, the bullet that fits the best to provide a smooth straight entry into the bore will be accuracy. Of course it depends on alloy, powder charge, etc. etc. etc.

[porkchop bob]

I agree with Charger in his reply #5 and with Joe B in his reply #4. The boolit has to be supported by the throat and guided concentrically into the rifling of the barrel. A cone shaped throat does neither.
Bob

[Phil]

One time I tried an experiment just for fun. I chambered a barrel with a reamer that had no throat section at all. Then I throated it with a throating reamer to just a bit shy of where I wanted to shoot the cast bullet I wanted to use in the rifle. Next I took a piece of barrel steel that had been bored and reamed but not rifled. I reamed it with the same throating reamer that I used to put the throat in the rifle barrel. Finally I threaded the exterior of the piece to 7/8X14 so I could screw it into my loading press, and made a set of punches to get the bullets into and out of the die. Thus, I had a bullet custom fit to the throat of the barrel it was being shot in. Short version is that it worked very well indeed. So well in fact that I made another die to work with an M1A for a friend and it worked very well also. The rifle shot exceptionally well with 311291's so treated and loaded with my old standard charge of IMR4198. That charge. by the way, works well in any full size caliber thirty to 8mm round with cast bullets. IE all the WWII era military rifle cartridges.

Merry Christmas all,

Phil

[Bass Ackward]

Maybe we should stop our love of the bore rider nose bullets and get some experience with groove diameter bullets.

My gun/s shoot pretty good when the bullet does NOT fit the throat, whatever the throat is.

joe brennan


Joeb,

I know you already understand a lot of this, but I will write in detail so others can follow.

I don't see any mystery to throats. The barrel, the bullet design, and the load still are responsible for achieving the accuracy. Because breach seating eliminates the transition from the throat to bore under pressure and remains the most accurate method for shooting cast. The job of a throat is to maintain the prefect conditions you strive to create from molding to the loaded cartridge.

And bore ride bullets serve a purpose and fulfill a need for guns with less than "ideal" throat conditions. But with bore rides comes the disadvantages that can and do alter perceptions of false limits on cast velocities that aren't really valid. Cast gets the reputation for velocity ranges and limits that they do because of all the different bullet designs and throat shapes and sizes. And the misunderstanding of how to minimize those effects from shooters who decide to use cast.

Bullet obturation from pressure is a fact. Obturation is an uncontrolled deformation of your precisely crafted bullet. Deformation occurs until a stronger metal stops the process. Exceede the ability of the hardness to resist obturation and you better have a throat design to support or better yet, minimize it. Thus some rifles can run soft cast at significantly higher velocities than others.

Where you see the difference in throats is when you apply pressure in excess of the hardness. The ideal situation, is for a fully supported bullet at the same diameter as a bore. Breech seating does this. The ideal throat would attempt to replicate this support until the slug can enter and be supported totally by the bore. But the bullet must still chamber. And .... this is a temporary condition at best.

Most cast benchrest quality throats are cut so that they are .0005 over groove. The problem is that anytime you cut, you leave tool marks. These marks will wear away quickly depending on the pressure a bullet is subjected too and the rate heat is applied to this area. These marks can easily take .0005 on a side to clean up. Thus, the .0005 throat that was cut opens to .0015 over bore or more depending on how sharp the reamer was, the type of steel, how fast it was pushed, etc. You could very easily lose .001 on a side and end up with a throat .0025 over bore. You can't cut without roughing to some degree.

So what happens is that a guy get's a new gun and slugs. He gets his measurement and records them, accepting these as fact and believes his job to be over. He makes his choices for bullet fit of a one diameter slug at that time. And depending on his chosen pressure levels, his throat cleans up over time and the leade angles change. He loses the fit he attempted to create in the first place and loses some level of accuracy. Odds are, he never slugs again to follow his throat out until it smooths and eventually stabilizes.

So a "perfect" thoat for cast is impossible to create or cut and breech seating will always remain king. Throats are a fluid and ever changing condition. So as a shooter you accept cast limitations and the velocity levels that come with those, OR you must learn how to fit .... or choke to achieve success at higher pressure levels or simply be happy in the cast velocity zone.

[fecmech]

Bass --could you please explain "breech seating" if you would. I have heard the term many times but am not sure what it really entails. Thanks Nick

[wills]

Breech Seating:
http://www.assra.com/getting_started.htm

[charger 1]

Would having a mold made that was identical to throat of that gun, filling said throat,and touching rifling ,used only in that gun not for the most part accomplish the same thing?

[Bret4207]

IMHO the throat area of the chamber, that area between the end of the cartridge neck and the start of the rifling, should be a guide for the boolit into the rifling. It should support and guide the boolit straight into the rifling without being so large as to allow the boolit to obturate beyond a reasonable limit. Whether this is a hangun or rifle it should act the same. If the throat is oversize it would allow the boolit to obturate more than it should and might allow the boolit to wander to one side or the other of the throat and become misaligned. Too small a throat either gives you "Ruger-itis" where the boolit is too small for the barrel or in a rifle won'
t allow you to seat the nose into the rifling lead.

Im my perfect world the throat of a rifle would be large enough to allow the perfectly fitted nose to engage the lead while the first drive band would be just contacting the tapered throat. In my real world I seat to engae the rifling if possible and run my boolits fat enough to fill the grooves or maybe up to .002+ over if I can or have to. I don't load real hot for rifles. In revolvers I've lucked out so far as all my throats are larger than my grooves so I go to throat size.

Is this about as everyone else follows things? FWIW- I am a fan of the Loverin style rifle boolits and Keith style or maybe I should say SWC style revolter boolits. Support the body and keep the nose no longer than needed if possible. That being said, I love the look of the bore riders but they don't always work for me.

[charger 1]

Thats my take on it to. "ruger-itis" I like that..Ya its a definate ailment, but can be cured with a 460-461 throating reamer

[Bass Ackward]

Would having a mold made that was identical to throat of that gun, filling said throat,and touching rifling ,used only in that gun not for the most part accomplish the same thing?


Charger,

Read Phil's response above. The answer is yes. Until the throat changed enough that fit was no longer perfect.

Most guys that shoot cast at cast pressures / velocities, never need that kind of fit we are addressing a "perfect" world. Nor would most notice if they lost it. Only people pushing higher pressure applications will ever notice the accuracy loss as they lose fit. Especially if they are shooting soft lead, like for hunting. Then you notice it right away.

When it first started on my 35, I was using a 210 grain bullet at 2400 fps. Sub MOA groups just started to open up quickly to 2". I seated out farther and accuracy came back to just above MOA, but not as consistent. Fliers became more common and were wilder. I always wanted to stay light to keep my velocity up and thus had to make a 220 grainer. It required I change the olgive taper a little. Accuracy came right back to sub MOA .... for about 200 shots. So I made up a 250 that I am still able to use as my leade angle has stabilized at this time. Took about 800 rounds to get to this point.

So this is why I would always recommend someone start smaller and shorter than your desired dimensions if you are building. It helps if you use a seperate throater to cut a leade angle that is cast friendly right from the start. Something less than 3 degrees for a straight case and 1 degree or less for a bottle neck. Then it will wear faster and stabilize sooner. Phil had the process right above.

Otherwise, you have to work with or chase what you have.

[charger 1]

In my particular situation of running hard cast ie 21bn+, the throat entirely full with a .5 degree taper, and ball powders noted for far less erosion I think I'll get sick of shooting it before the accuracy is in the toilet

[Bret4207]

BA- How long did it take to see a change and do you believe the change is due to wear from powder errosion or actual mechanical wear from the boolit?

[Bass Ackward]

BA- How long did it take to see a change and do you believe the change is due to wear from powder errosion or actual mechanical wear from the boolit?


Bret,

My barrel took 180 rounds of jacketed to get things smoothed up enough to shoot HV with soft lead. Then I got maybe 200 rounds with the 210 grain before things moved. I consider that fairly soon. Then maybe 200 rounds with with the 220. I was getting nervous. Now I have at least 800 rounds, but I just fire lapped this gun again using the Tubb's System to decrease the tapper even more and correct any small dimensional abnormalities that remained. I find this system preferable in that it is faster and cheaper to shooting several hundred jacketed now.

The purpose was to see if it improved any for the use of soft lead and then if so, how or why. Because this barrel glistened before I started! You know, another danged experiment.

As to mechanical wear, I can tell you this. I was using a plastic wire nut and 600 grit to clean up bad tooling marks in a forcing cone on a Redhawk that was causing or destroying plain based bullet bases and causing leading. When I was finished, you could visibly see the clean, 600 grit, spiral swirls, while very slight, looked rough compared to the gleam of the lead polished bore. I thought to myself, this was going to take a couple of hundred shots to polish out to get to the same bore like condition. Do you know how many shots it actually took?

Six! One cylinder full of soft, 14 BHN bullets at 20,000 psi and CSI couldn't tell the forcing cone was ever roughed up. So if lead can polish 600 grit marks that fast, imagine that very slight errosion from powder is polished away with each lead shot. The polishing action will be highest, where pressure peaks and bullet obturation is most severe.

So my answer is that all wear is powder errosion, and the lead just polishes the damaged surface. Chicken or the egg?

[Bret4207]

Bass- You tickled a memory- I recall a gun that I handled/saw/owned/worked on (who knows?-CRS) that had an unusual chamber. A couple of the lands appeared to stick out further to the rear of the throat than the others. I suppose they could have been a bit higher or lower than the the other lands. When it was chambered the reamer didn't hit them the same as the others, or maybe the lands weren't concentric with the grooves. All I recall is the 2 lands extending way beyond the other. I wonder if this would cause the same type of change you talk of? The wear would be accerlated on those lands I would think. Interesting question, for me at least.

I also like the fire lapping idea having spent what seemed like years on the end of a lapping rod in my youth. I have the gear to do it and a couple rifles that would likely benefit. Time, I need time...

[felix]

Indexing rounds will exhibit what you see, Bret. The guy who owned the gun beforehand prolly had to do this to get any accuracy at all because of some misalignment. ... felix

[joeb33050]

Joeb,

Because breach seating eliminates the transition from the throat to bore under pressure and remains the most accurate method for shooting cast.

(I don't know this. I'm pretty well convinced that breech seating was invented because the straight tapered cartridges of the time had no necks and couldn't be reloaded with accuracy. 32/40, 38/55, have no neck. Thus the only way to get accuracy is to breech seat.
The ASSRA guys and the CBA guys shoot under different conditions. I wonder how the ASSRA guys would do if they had a certain 15 minutes to shoot 10 record shots.
I'm not saying it ain't true, just that I know of no proof.)

And bore ride bullets serve a purpose and fulfill a need for guns with less than "ideal" throat conditions.

(I'm starting to think that Harrison was right, and that we like bore ride bullets because they "should be" more accurate. I'm having a lot of luck with straight .309 bullets without the nose. Maybe I'll never know, bore riders have worked for me for many years-but I'm losing faith.)
joe b.



I also don't know how to reply to points in a post. I have to add something here.
joe b.

[45 2.1]

I think we would know a lot more if we got people to make impact castings of the "throat", got somebody to draw the picture, and then watched how different bullets/diameters shoot. If we had a library of these maybe we could learn something about this "throat"business.
Remember Harrison's warning about nose riding bullets and small lands, Maybe we should stop our love of the bore rider nose bullets and get some experience with groove diameter bullets.
I wish I could do the CAD drawings to keep records with, I think this would be worthwhile.
My gun/s shoot pretty good when the bullet does NOT fit the throat, whatever the throat is. joe brennan

CAD is not to keep records with, it IS very nice to show what the relationship of a boolit to the guns throat though. I've done many throat slugs and designed boolits to fit them, but relateing that to you means that you would need CAD to see it and to understand just where on the boolit it needs to fit. I've done this with a couple of people and they're still learning. The sad truth is most molds produce grossly undersize boolits. Your bands and nose bore ride feature is a joke on them. You need big enough bands and a bore engraved nose to see any benefit. If you could see and feel how one is supposed to fit and see it shot, your eyes would open wide and you would be astounded.

[joeb33050]

CAD is not to keep records with, it IS very nice to show what the relationship of a boolit to the guns throat though. I've done many throat slugs and designed boolits to fit them, but relateing that to you means that you would need CAD to see it and to understand just where on the boolit it needs to fit. I've done this with a couple of people and they're still learning. The sad truth is most molds produce grossly undersize boolits. Your bands and nose bore ride feature is a joke on them. You need big enough bands and a bore engraved nose to see any benefit. If you could see and feel how one is supposed to fit and see it shot, your eyes would open wide and you would be astounded.

I'm suggesting a set of drawings showing the neck of the case to the rifling, the ? throat?, taken from impact or cerrosafe or sulfur castings. Along with these could go actual bullet drawings and accuracy results. In this way we wouldn't get bogged down with the words, the ball seat, leade, throat business. Maybe pictures of the bullet in the ?throat? as far as it will go. A picture, I'm told, is worth a thousand words.

joe brennan

[45 2.1]

I'm suggesting a set of drawings showing the neck of the case to the rifling, the ? throat?, taken from impact or cerrosafe or sulfur castings. Along with these could go actual bullet drawings and accuracy results. In this way we wouldn't get bogged down with the words, the ball seat, leade, throat business. Maybe pictures of the bullet in the ?throat? as far as it will go. A picture, I'm told, is worth a thousand words.

joe brennan

Not in this case, a normal view shows nothing. You would look at a normal view at actual size and the area of interest are like a decimal point on the drawing. You have to blow that small area up and look at a segment of the boolit that is about .02" long in the view to actually see what is going on. Normal drawings or pictures don't work well for that. Then you have to have someone who knows what is going on explain the fit issues at those points. I've been through this with a couple of people that do have CAD and it isn't to easy even then to educate them as to what they are looking at.

[Bass Ackward]

I don't know this. I'm pretty well convinced that breech seating was invented because the straight tapered cartridges of the time had no necks and couldn't be reloaded with accuracy. 32/40, 38/55, have no neck. Thus the only way to get accuracy is to breech seat.joe b.


Joe,

There is a difference between "why" breech seating was invented and the results. Why couldn't you breach seat a 32 Mag with no throat. Or a 7.62X39?

The purpose of a throat is simply to guide a bullet into a bore under very harsh conditions. Only steel can support deformation under pressure. Breech seating allows lead displacement of the rifling under much better conditions so that the only thing the bullet has to over come is it's own inertia when the case is fired and zero obturation takes place. Just picture it in your mind.

Then let's go to making drawings of a throat. How can you have pictures and drawings of an ever changing throat? At what point do you do this? Remember your Savage 223. How many shots did you have before it started to change? Do you think that all dimensions remained the same? How do you "perfectly fit" an ever changing throat with a one diameter bullet? And let's stay with a Savage, each throat on a brand new rifle can be different in diameter, length, and angle depending on where it was made in the reamer life cycle.

So in a way, "fitting" a throat on a single gun is impossible. (I left this by itself because I know it is going to cause some grief.)

Bullet fit really amounts to "filling" a throat because each one is different and changing. And the change from that point on depends upon cleaning methods, antimony levels in the mix, pressure ranges and rates of fire, how large a bullet you are trying to shove down that hole, etc, etc. (Yes. Throat wear is accelerated by choking!)

You can only fit a one diameter bullet to one throat and only for a short time unless you shoot pure lead and tin at low pressures and velocities. Now can you see why bore rides bullets are so popular? They are designed for the bullet to be the guide and provide alignment by a much "more consistent" bore because the throat IS CONSTANTLY changing. So the best thing that you can do is seat out and chase the rifling.

And this is also why choking is so successful. Diameter of the throat is the least changed dimension. So it is way longer in time before someone who chokes "understands" (or cares) what is happening. In fact, if that person reslugged, he may become sick. :grin: The guy shooting .309 bullets lives with it all the time. Huh?

[45 2.1]

So in a way, "fitting" a throat on a single gun is impossible. (I left this by itself because I know it is going to cause some grief.)

Ha, not this time. This is the stage your at John. Keep plugging away and you'll get it eventually. Throats do change, but not as drastically as you say.

[felix]

Bob, throat wear is all pressure (heat actually) dependent. Tighter the fit under dynamic conditions (obturation) the more heat generated. About a 1000 (or less) of 65K CUP rounds is about what it takes to contemplate a condom BR barrel change. Shooting high antimony boolits, I bet, will probably take half that number of rounds to notice the same change at the same pressure. However, in practice the same pressure might not occur with a different projectile shape/composition. Shooting at 65K CUP would be a very strange requirement for us lead slingers, unless trying to force a throat change. ... felix

[Bass Ackward]

Ha, not this time. This is the stage your at John. Keep plugging away and you'll get it eventually. Throats do change, but not as drastically as you say.

Bob,

Dan at mountain molds has documented on his site, his experiments where he started with .311 diameter bullets weighing 160 grains that "filled" his throat when fairly new. Then ended those experiments less than 12 months later where he decided he needed a new barrel. At that time he was shooting .318 bullets in a .308 bore and needed 180 grain bullets to reach the rifling. Maybe 1000 rounds total.

Benchresters all over lose throats at about 1000 rounds and still have the original tool marks in the barrel. And they start with ULTRA minimum throats. Stainless and chrome molly in my case. Makes absolutely no difference to wear.

I consider that proof. And that qualifies as MAJOR movement from underbore cartridges. And no CAD program or amount of tweaking is going to make a bullet fit with those dimensional changes.

I once read an article that talked of lever companies try to get 200,000 chambers per reamer on these straight cartridges. Try drilling 200,000 holes as fast as they do chambers and see how your bit holds up.

So imagine how different new factory guns are between each other depending on when they were made. Unfired!

And this board is full of examples. Take JohnH with his 375JDJ. Started with a .377 slug to choke and about 800 rounds later could chamber a .379.

Read the statement that you always post at the bottom of your posts. Great quote. Is it me that is naive?

[45 2.1]

I guess the CBA guys are full of it then, along with most of the worlds arsenals from different contries relating barrel life. Since when does cast use 65K chamber pressure. Boolit alloy materials are hard and coarse at times, that along with pressure and erosiveness of burnt and unburnt powder washes a throat, but that is not saying that it enlarges the throat, just that it washes the lands away. I bet none of you actually measures a throat and plots it to the ten thousandth and checks it every thousand rounds to see what happens. The throat erosion is not a straight line plot in regards to pressure by the way. As far as your boolit size and fit issues, there is more going on there than you realize. When you start measureing things closely, you'll find that out for yourself and won't speculate on it. I'll stick with the folks that actually do this seriously thank you.

[felix]

Bob, if bullets/boolits only wash the lands away only, then proper obturation (defined as any boolit deformation here) did not take place. Period. I don't care what the other guys say. It is really a simple concept. ... felix

[45 2.1]

Bob, if bullets/boolits only wash the lands away only, then proper obturation (defined as any boolit deformation here) did not take place. Period. I don't care what the other guys say. It is really a simple concept. ... felix

You might want to check out past NRA articles about the Finnish match rifles and what they related about them. Very interesting stuff. Not everything is what it appears to be. A lot of arsenals check this kind of thing. Since this site is not really concerned much about accuracy, I think any benchrest related barrel life is moot since quite a few of those bum barrels are then used for varmit guns or put up on cast boolit rifles and provide admiral service (MOA or less) for at least another 3,000 rounds or more.

[felix]

Bob, actually more than 3000 rounds. Try 10,000 rounds at least for a half inch at a hunnert using 35-40K CUP. Take-off condom BR barrels are the cats meow provided they are fitted like you say with a boolit form than can be constantly seated longer as the wear cone enlongates. Better yet, change the boolit form every so often for even a better resistance, but that to me is overkill because it is more fun to change the alloy and pressure if there is enough boolit length available. ... felix

[45 2.1]

Felix-
Now that was what I was trying to relate. Throat erosion occurs moreso with the lands than actual throat diameter. This happens even with jacketed. Much depends on the pressure along with the "heat of burn" of the powder used. I have 100 year old militaries which have almost no lands left which, if fed the right boolit, will shoot MOA or better (when I can see the sights). A lot depends on just what you do.

[felix]

In summary, you have to "pre-obturate" a boolit for an accuracy fit, or let the barrel and boolit reshape themselves together, hopefully in unison, over time. In either situation, the idea of indexing cartridges will guarantee an out-of-round throat in almost no time at all. ... felix

[joeb33050]

Joe,

There is a difference between "why" breech seating was invented and the results. Why couldn't you breach seat a 32 Mag with no throat. Or a 7.62X39?

The purpose of a throat is simply to guide a bullet into a bore under very harsh conditions. Only steel can support deformation under pressure. Breech seating allows lead displacement of the rifling under much better conditions so that the only thing the bullet has to over come is it's own inertia when the case is fired and zero obturation takes place. Just picture it in your mind.



We have records of single shot plain based bullet single shot breech seater guys vs. Production, Heavy and Unlimited class fixed ammo guys at CBA national matches, all shooting under the same conditions. I've just spent ~4 hours digging into 2005, 2006 records.
I took out the long range handgun guys and anyone who didn't finish all group matches. 100 and 200 yards, 5 and 10 shots, calculated the averages by class and the smallest group by class.
Here it is, I hope:
2005 CBA Nationals
Summary PBB PRO HVY UNR
Average 5/100 0.631 1.290 0.772 0.728
Smallest 5/100 0.194 0.236 0.356 0.251

Average 5/200 1.750 2.585 1.952 1.900
Smallest 5/200 0.742 1.010 0.594 0.786

Average 10/100 1.009 1.629 1.162 0.978
Smallest 10/100 0.580 0.769 0.614 0.323

Average 10/200 2.951 3.214 2.486 2.553
Smallest 10/200 1.391 1.633 1.148 1.332

2006 CBA Nationals
Summary PBB PRO HVY UNR
Average 5/100 1.009 1.629 1.162 0.978
Smallest 5/100 0.580 0.769 0.614 0.323

Average 5/200 0.710 1.357 0.847 0.855
Smallest 5/200 0.186 0.495 0.151 0.259

Average 10/100 1.806 3.093 1.942 1.870
Smallest 10/100 0.882 1.075 0.742 0.482

Average 10/200 2.204 2.846 2.106 1.850
Smallest 10/200 0.723 0.692 0.571 0.475

Gopd help me, but I do love data so!
joe brennan

[felix]

Joe, what does this data tell you? ... felix

[PatMarlin]

So the beautiful thing I'm reading here is even after several thousand rounds fired, the cast boolit shooter can adjust and compensate, and still keep on shootin' accurately, whereas condom shooters are condemed to a barrel change out.

Right?.......:Fire:

THis means a never ending challange and fun for cast bolliteers.. :drinks:

[felix]

Very true, MJ, but only in the BR condom world. When a boolit design shoots, it shoots, and the gun must be made commensurate. Changing the BR bullet is NOT in the cards. The barrel twist is intentionally set to a minimum, so a bullet design change won't work (unless luck is involved). Condom BR guns are meant to be shooter competitive only, seperating the weather readers from the rest of the crowd. All of these guns will typically shoot the same group in an ideal environment. ... felix

[45r]

I dont know about rifles but revolvers like a perfect throat dimention.Ive firelapped several and some only needed 12 rounds (cleaning the barrel every shot)because they had perfect fit (same throat barrel groove diameter) and others 30 rounds and others 72 (ruger stainless).Somewhere along this process the revolver starts shooting very well with leading reduced to the point you can shoot without no irritating build up in the forcing cone area.The improvement in accuracy is well worth the time and labor.The only revolver I have not done this to is a model 83 but it shot inch and a half at 50 and didnt think Id could improve on that.The quality was so impressive I decided to leave it the way it came from the factory.It always amazed me how some revolvers started shooting almost as good as the expensive F/A after the right amount of firelapping.Also I dont need to use harsh bore cleaners because the bore is so smooth and cast bullets shoot with out leading even plain base with the right load.It sure is good to not see any lead in that forcing cone area when you get done cleaning your wheel gun.

[Bass Ackward]

Forget trying to tie in accuracy at this point. I think the discussion of throats and wear is valid regardless of the position you eventually take. If we don't understand wear, how can we ever fit?

Bob,

I gave three different examples for cast. High, medium, and low pressure that caused different wear patterns.

Dan's work was HTWW at full pressure loads. Rifling did lengthen, but how much isn't written. Dan is a choker. Ball seat diameter opened .006 from choking as it tapered. Dan made several bullet designs chasing fit.

My exqample is with 14 BHN at 28k-36k .001 over bore. 800 rounds and the throat "did not" open much. My diameter was cut .360 and is now @ .3605. Obturating pressure isn't reached for almost 1 1/2" with SLOW powders peaking at 2", well after the bullet is moving and supported by the bore. But the throat has lengthened from .250 ball seat to .340 before the rifling starts. And it has stabilized for now. But that is almost .100 movement from the origional. With medium hard cast, and in @ 800 rounds.

JohnH shot soft lead with fast pistol powders at low pressures. Quiet loads he called them. In other words, he was shooting cast like cast are normally supposed to be shot. His obturation was again pretty much in the throat and it opened in that area. With @ 8 BHN lead lead if I remember correctly.

Three different .... senarios under which wear took on different patterns based on different reloading / shooting techniques. So erosion until stabilization is real and pretty much uncontrolable. From my situations, once a throat wears to a certain point, the throat does stabilize and fit from that point can be done. But again, based upon method of useage. The throat will change dramatically based on how it was shot in. Change the shooting method and the wear pattern can begin again until the new pattern is established and stabilizes.

So I guess you could say that you get less wear in a sloppy throat than a tight one. And I guess you could say that you get less wear from a tapered throat than a tight ball seat as the steel is already away from the heat. Unless you choke. But fit until any stabilization has to be fluid. And the shape of a stabilized throat is going to be unpredictable based upon everything discussed here.


Joeb,

I don't understand the point you want to make with your data?

The throats shot at CBA events probably come as close to "short and tight" or "supported" (like breech seating) as you can get without actually breech seating. Sort of makes my point doesn't it?

[Bass Ackward]

So the beautiful thing I'm reading here is even after several thousand rounds fired, the cast boolit shooter can adjust and compensate, and still keep on shootin' accurately, whereas condom shooters are condemed to a barrel change out.

Right?.......:Fire:

THis means a never ending challange and fun for cast bolliteers.. :drinks:


Pat,

Correct. But adjust you must or accept mediocrity.

Fit is not a one time process. It is continuous. Often why one guy can't shoot a Lyman design 311XXX for crap and 5 years later tries it again only to have it cut holes.

A big advantage to bore ride designs that fit is that they fit for a very long time using the bore and ignoring the throat. As opposed to a one diameter bullet that uses the throat. Assuming it isn't breech seated.

[joeb33050]

Joe, what does this data tell you? ... felix

PBB class is single shot rifles, breech seated.
Production class is "almost" stock rifles, fixed
Heavy class is "not stock" rifles meeting a weight limit
Unlimited class includes rail guns, also fixed
The Production class has most shooters, PBB has few, total well under 50 for allo classes.
5 shot groups are shot in two sets of two, total four
10 shot groups are shot in two sets of one, total ywo
Everybody shoots the same match at the same time.
It appears to me that PBB breech seated bullets are "more accurate" than the other classes shooting fixed, but there's no certain way of deciding.
If I apply 1 for first, two for secong three third and four fourth to each average, and sum the scores, PBB wins.
Other measures are possible, maybe better.
I begin to suspect that Bass is right, breech seating is more accurate.
I'll do the other two years of data that I have.
Also, compare the 10 shot 200 yard PBB groups to the ASSRA records, that show Jim Borton shooting groups ~.750 for 10 at 200 in ASSRA matches.
It looks like in 2006 the CBA shooters shot smaller goups than the renowned Rowland group, under .750 for all classes.
I'll have to check this. Can this be true?
Years ago Al Windsor, a well known SS shooter and collector, said to me, about the CBA: "They don't shoot much better than the SS guys shoot".
Anyhow, here's the data, I'll check those suspicious 10/200 groups, make of it what you will.
joe brennan

[felix]

Pure breech seating works accuracy wonders. That has been shown over time, and most especially during the VietNam era when "football fields" were continuously hit 20-26 miles away from BigMO. Somebody want to compute the minute-of-angle? Remember, all sighting was done by computers, so let's assume no errors in that department. ... felix

[joeb33050]

I screwed up on the first one. This has been all re-done, any differences checked. I think it's right.
2005 CBA Nationals
Summary PBB PRO HVY UNR
Average 5/100 0.631 1.290 0.782 0.769
Smallest 5/100 0.194 0.236 0.356 0.251

Average 5/200 1.750 2.585 1.952 1.960
Smallest 5/200 0.742 1.010 0.594 0.786

Average 10/100 1.009 1.629 1.162 1.014
Smallest 10/100 0.580 0.769 0.614 0.323

Average 10/200 2.951 3.214 2.486 2.589
Smallest 10/200 1.391 1.633 1.148 1.332

2006 CBA Nationals
Summary PBB PRO HVY UNR
Average 5/100 0.710 1.357 0.847 0.855
Smallest 5/100 0.186 0.495 0.151 0.259

Average 5/200 1.806 3.093 1.942 1.870
Smallest 5/200 0.862 1.075 0.742 0.482

Average 10/100 0.975 1.648 1.201 1.028
Smallest 10/100 0.662 0.688 0.558 0.420

Average 10/200 2.910 3.909 2.947 2.600
Smallest 10/200 1.356 1.284 1.474 1.141

While some numbers are different = correct, the picture remains the same. PBB, by at least my measure, seems to be the most accurate class.
At the ASSRA matches Jim Borton has several times shot 10/200 in under or ~ .750". Nobody at these CBA matches has come close. But, ASSRA means re-entry and wait out the conditions.
Again, I'll go through 2004 and 2003 and we'll have more DATA!!
joe b.

Can somebody get somebody to take out the/my incorrect post? Thanks.

[Ricochet]

most especially during the VietNam era when "football fields" were continuously hit 20-26 miles away from BigMO.

I sure wouldn't want to be within a football field length of one of those shells' impact!