Obturation?

[KYCaster]

So, I'm sittin' here, impatiently waitin' for Santa to come down the chimney with Peace On Earth and Good Will Toward Men in his bag of goodies and started thinkin'.(I know....lay off the Egg Nog) Anywhooooo, there's another thread running about "New use for a gas check" or something like that...what does a GC do and how does it do it? Several of the responses mention "obturation" and the GC's effect on it.

Now the term obturation has been used quite often in lots of different discussions on this forum and I think most everybody understands the concept...because of the pressure of the combustion gasses, the boolit expands to fill the available space in the throat/barrel. We even have a formula to predict the optimum ratio of pressure to alloy hardness.

I don't think anyone denies the existance of the phenomenon, after all there are boolit designs that make very effective use of the concept.(Mini ball, hollow base WC) But...there are some things that I don't quite understand about it.

Let me say first that I have been shooting lead boolits in handguns for more years than I care to remember, but I've only been loading lead in rifles for three years or so. I've seen the effect of under sized throats mated with oversize barrels in revolvers, and I think I understand the role that obturation plays in loading for handguns...but I'd like a better grasp of what is happening in my rifles.

So...my first question...if you size your boolit .001-.002 over groove dia. as is most often recommended, how can it "bump up" to fill the barrel since it's already larger than the available space. If you ignore the groove dia. and size to fill the throat as is often recommended then the boolit often is even more oversize.

Another theory I've seen in print more than once says that the boolit will expand to fill the throat then be "swaged down" as it enters the rifling, thereby creating a "perfect fit" in the bore, assuming that the alloy is compatible with the pressure of the cartridge. If that's the case then wouldn't the instantanious obturation of the boolit interfere with the brass case releasing the boolit and cause excessive pressure?

Can any of you guys shed some light on this for me? I've obviously got waaay too much time on my hands. I oughta be out in the shop casting boolits or sumpthin rather than sittin here waitin for Santa to show up...only thirty five minutes to go!!

Hoooo, Hoooo, Hoooo

MERRY CHRISTMAS TO ALL!!!!

Jerry

[The Double D]

I've rattled this idea around on my brain a bit also. It would seem if the obturation took place in the neck you would see pressure signs sooner or later.

I have been led to believe that this obturation takes place not in the case mouth but in the throat when the bullet hits the rifling and meets resistance. Prior to that the lead bullet is just so much ejecta. Well maybe not that simple, but you get the idea.

Doesn't Mann do some work on this theory? My copy is 7000 miles away in storage.

I can't but feel that there is limited benefit to be gained from obturation. Even then things are going to have to be near perfect to make it work. The bullet is going to have to be a close fit to start with. It's going to have start square to the line of bore and stay that way. Misalignment is going to be exaggerated by obturation.

I am retiring in thee months and I see I am going to have crack the books; The Bullets flight, Hatcher's Notebook, The Gun and its Development, Sharpe's Reloading. No modern author other than Charles Dell has touched these subjects so well.

[NSP64]

I believe that obturation takes place with soft(pure) boolits that are undersize for loading purposes(muzzleloaders) Hard cast that is oversize won't obturate. hard cast that is undersize will flame cut the lead in the barrel. The initial (shove the base through the nose) pulse of ignition causes obturation. AS I understand it anyway.:drinks:

[NSP64]

Also the older revolvers using Pb could obturate when the boolits jam into the forcing cone, as long as the barrel wasn't to far oversize.

[felix]

""""If that's the case then wouldn't the instantanious obturation of the boolit interfere with the brass case releasing the boolit and cause excessive pressure?"""

Yes, you are correct. Tight fitting necks have to be taken seriously. Allow 001 total clearance for all "boolits" and you will be safe using the correct powders for the cartridge at normal working loads. Notice: This has nothing to do with bore/groove size, but NECK size.

felix

[VTDW]

Wasn't there a gentleman that proved that the base of a boolit travels faster the the nose of a boolit for the first 10" of barrel?

Dave

[Loudenboomer]

Obturation ? Isn't that what women do about every 28 days??? :-D

Seriously -- Good thread!

Merry Christmas All

[Bass Ackward]

So...my first question...if you size your boolit .001-.002 over groove dia. as is most often recommended, how can it "bump up" to fill the barrel since it's already larger than the available space. If you ignore the groove dia. and size to fill the throat as is often recommended then the boolit often is even more oversize.

Another theory I've seen in print more than once says that the boolit will expand to fill the throat then be "swaged down" as it enters the rifling, thereby creating a "perfect fit" in the bore, assuming that the alloy is compatible with the pressure of the cartridge. If that's the case then wouldn't the instantanious obturation of the boolit interfere with the brass case releasing the boolit and cause excessive pressure? Jerry


Jerry,

Real complicated. See If I can make it understandable. Just keep in mind that a certain metal hardness will require a certain amount of pressure before it begins to obturate.

The biggest difference between handguns and rifles is that you can't fit a bullet for as fast of a seal as well in a revolver and you get more blow by gases before an actual seal takes place. Powder speed not only controls how much pressure, but how fast that pressure comes up.

Take my Whelen with 2400 and RL15. 24 grains of 2400 is @ 27,000psi. That 27,000 psi peaks in .9" after the bullet starts moving. The same 27,000 psi peaks with RL15 after 1.99" of bullet travel. The obturating force on the base is higher with 2400 that only produces 1750 fps. While the pressure on the base with RL15 allows 1900 fps. So I can actually go up to 30,000+ psi before my ACWW hardness begins to collapse from too much pressure, so I now get 2100 fps.

These two things work together with the first statement to dictate 1. how much lube you need on your bullet. and 2. how good of a lube that has to be. Because people do not understand these things they search endlessly for the "perfect" lube when along comes someone else that makes pure bees wax do the same thing without leading. This is why one guy can shoot LLA and another can't. This is why another guy might prefer a super hard lube to another wanting a soft lube.

If you can keep those two things straight in your mind, then obviously different things are happening at different times under different conditions. As a caster / reloader you are attempting to prevent, moderate, or take advantage of those things to make them work for you as you describe with undersized handgun throats to create a seal before your lube is blown away.

Coking on the other hand is a strengthening trick to prevent leading in rifles and it is an alignment trick too. But it is also a seal trick that prevents lube from being blown off where it does no good. So when you read of someone advocating choking or larger diameter bullets as a method to preventing leading, it is safe to say that he shoots with a style that causes too much blow by before he gets a good seal and choking solves his problem. If you seat out and seal at the front, then a seal is a seal and leading is prevented because your bullet carries all that it needs to accomplish the mission.

So what kind of obturation, which will eventually turn into deformation you get, either in a handgun or rifle depends on how fast and how hard you spank that slug. If that pressure occurs when steel isn't supporting the lead, then you are going to obturate. If you obturate when it is supported by steal, either your lube has failed or you are compressing / deforming your bullet and you must either 1. cut pressure, 2. go to a harder slug, or increase pressure so that the collapse is most uniform.

Often people stop looking for accuracy when the bullet first starts to deform when they may be successful going on up. This is how, along with a square groove bullet design, Elmer Keith was able to shoot an 11 BHN bullet at 34,000 psi when the 4122 X hardness chart says 11 BHN should crap at 22,000 psi. Handguners generally understand this better than rifle shooters because one confusing variable is left out ..... RPMs.

Rifle shooters reach this stage of obturation at HV and because they are getting higher velocity call this the RPM ceiling. The faster the twist rate they have, the faster the powder they use, or the heavier the bullet they have, the sooner they reach this bullet deformation point. If they have a good bullet design that will collapse uniformly or a lighter bullet, they can go on up with pressure too and achieve better accuracy than they expect if they can hit a harmonic node with their barrel, bedded as it is. These RPM believers stop short of their true HV / accuracy limit. This is why you always see me argue against this and say, it ain't RPMs, it's pressure.

Help or confuse?

[felix]

Accuracy is a function of RPM/vibes/atmosphere/alloy; RPM is a function of velocity; velocity is a function of pressure. ... felix

[twotrees]

I was taught , in Physics, that all things being equal, a lighter object will accelrate faster than a heavy one.

That being the case, could it be when we fire a Hard boolit in a firearm chamber the gas check being much lighter than the boolt it slams forward into the base and further helps the slump/ obtraution process.

I was also taught that the base of a boolit starts moving before the nose so the base will get bigger, even with a hard boolit, until the push takes over the whole boolit. Lead and it's alloys creep at room temp (will deform under pressure without breaking) if given time. When you use a fast powder the Time to deform is reduced. A slower powder will cause the process to act longer , therefore more deformation.

I know about the RMP limit but could that not be, just this Creep process. caused by force over longer time?

What we call hard really isn't hard at all, in the real world, so all our boolits will have a creep limit.

How do we get around this?? Shoot harder alloys, add silver (Werwolves?) ,slow down the boolit and increase the speed of the powder burn. BPCR shooters have good luck with low alloys and Black Powder. BP burns VERY fast so the deformation takes place in a short time then goes way down. Even 2400/Unique are slow compaired to BP, so we get more creep in the boolit.

All I'm asking is if we have been missing this important property of lead alloys, could we do anything that would use it rather than ignore it and keep sizing our boolit larger and larger??

Sorry havn't been to work since Friday and I'm trying to get into wind down mode instead of R&D mode.

TwoTrees.:castmine:

[Larry Gibson]

Argue all you want Bass but in a way we do agree. Pressure causes obturation. Too much pressure or pressure applied too abrupty causes the bullets to deform in ways that unbalance the bullet. We agree on that and we agree on what it takes to reduce that unwanted obturation. What we disagree on is it is those unbalances in the bullet that are affected by RPM. At a certain RPM level (not a "limit" but a threshold of several thousand RPM) causes the bullet to be less accurate in flight. All cast bullts are adversely affected by RPM during flight at a certain level. You can up the level by using slower burning powders, using a harder alloy, using a shorter bullet with smaller lube grooves in a given twist barrel. However, when using a common mould none of those are going to get accuracy as good at 2400 fps than they get at 1800 fps in a faster twist or 9-10". Example; a 311291 in a 10" twist '06. Also it is well proven that accuracy will still be better even when using a custom designed bullet such as your LBT 154 gr bullet all "noded" out in or below the RPM threshold. Not to say one can't get decent accuracy at high velocity doing all the tricks you mention but the point is; accuracy will always be better in or below the RPM threshold with any cast bullet.

BTW; Elmer disigned his bullets with the square lube grooves to hold the lubes (in those days the lubes were not anywhere near as good as they are today, none held a candle to 50/50 alox/beeswax) used in the grooves to prevent leading. It was the leading caused by insufficient lube in those days that caused the inaccuracy loss with his magnum type loadings of the .38, .44 Special and .45 Colt. These days many of us regularly drive round lube grooved 429421s with the same alloy (1-10 tin/lead) out of .44s at the same velocity with the same 22 gr of 2400 and with the same accuracy that Elmer did with his square lubed 429421s. We just don't get leading to destroy the accuracy is all.

Merry Christmas BTW.

Larry Gibson

[Ricochet]

I don't believe a tightly fitting lubricated lead boolit/case neck/chamber combination causes undue pressure rise as it can with jacketed bullets, at least as long as there's no crimp in the case mouth with grease, oil or such around it. The boolit will not bond to the surface of the brass, and even if it's snug it'll slide straight out with the brass in contact with it. If there's a turned in crimp at the case mouth, and if lube, grease or oil fills the space between the crimp and chamber wall, it can resist the crimp's flaring out to release the boolit so that it has to be swaged down some to pass through it. Whether any of this is really significant compared to the force to swage the boolit into the rifling would be a matter to be determined by experiment.

Most modern bullet jackets are made of gilding metal, 95 Cu/5 Zn, or commercial bronze, 90 Cu/10 Zn. Both are brasses, cartridge brass being 70 Cu/30 Zn. They're similar enough that if clean bullet jackets are left in intimate contact with clean case necks, there can be some diffusion of metal atoms from one to the other, effectively "welding" them together at many small points. That's hard to break with a straight shearing force, but if the case neck can expand just a little it nicely breaks the bullet loose.

The classic info on case neck expansion and pressures is found in Hatcher's Notebook. Back in the '20s it was popular to lubricate the exposed ogives of jacketed bullets for target shooting with automotive grease. Hatcher discovered that when too much grease was applied it filled the space between the case neck and chamber wall, preventing case neck expansion, and this raised pressures significantly. There were some accidents when this effect was combined with tin plated bullet jackets, a measure that was used for a few years to reduce fouling from the copper-nickel jackets they then used. The tin over time produced a strong "solder" bond between the copper-nickel jackets and brass case necks, and unless the case neck could expand to release the bullet, pressures went way up. Hatcher even found a bullet downrange with the case neck still attached. It had torn off and gone through the rifling!

I don't believe this will happen with lubricated lead bullets, especially in reloaded cases that have the inside of the neck coated with sooty deposits. But I don't have the pressure testing equipment to prove it. I do load some combinations that fit snugly in the chamber. I believe as long as they're worked up that way, I have nothing to worry about.

[beagle]

Man, seems like we go through this ever so often. I think that we sometimes get obturation confused between revolvers and rifles.

Revolvers are different. When you light off a round, you get chamber obturation, with some blow by of propellant gasses. If it's of a correct hardness and of sufficient size, you'll get sealing in the chamber and I like revolver bullets soft and big to accomplish this. Once the bullet starts to exit the chamber and enter the forcing cone, another transation begins where the bullet either enters the forcing cone straight or missaligned. If straight, less deformation will occur and you'll get better accuracy. If missaligned, pressure will force the bullet to take the form of the rifling. While all of this is taking place, gas is escaping between the cylinder gap and also blowing by the bullet. If an imperfect fit, a small amount of lead is erroded and you get leading in front of the throat. Once the bullet takes the rifling, if the bullet is soft enough and large enough, almost complete obturation will occur and the gas is effectively sealed except for the cylinder gap. Some "skidding" will occur when the bullet takes the rifling. This will be minimized if the bullet is soft enough, big enough and the cylinder/forcing cone dimensions are within tolerance. It will occur to some extent because you're throwing a fast, soft projectile into slightly curved rifling.

Rifles are simpler as there are less constrictions to go through. In a rifle, you light off the powder. If there is sufficient neck clearance, the case mouth expands and a small amount propellant gasses escape by the bullet. The bullet moves forward to take the rifling. The bullet/throat fit allows more propellant gases to escape by the bullet and this is where we get some of our leading. When the bearing bands take the rifling, they cause resistance and the bullet starts to obturate. At this point, gas is still blowing by and taking lubricant and minute lead particles with it. As the bullet moves into the rifling, in the case of a PB bullet, an almost perfect seal will result, if the diameter is correct and the alloy is of a hardness that allows the bullet to upset and flow and if the bullet base is perfect. On GC bullets, the GC will encounter the rifling and being of copper alloy, it will provide more resistance than the lead and will provide an almost perfect seal as you get with a jacketed bullet. The bullet has now obturated to take the rifling and you have a good gas seal, good rifling contact and if a good bullet design, good accuracy.

One trick that us lead bullet shooters use is to seat the bullet into the rifling. This starts the bullet nose into the rifling and also provides quicker resistance for quicker obturation.

At times, bullet design will allow the noses of the bullets to slump during obturation. Case in point is the 311413 Lyman which got a bad name as the boys tried to use too much 4895 to drive them to service specifications. As a result of too much pressure, the alloy failed, the noses slumped and you got poor accuracy. Now I love the 311413HP and as long as it is kept below 2,000 FPS, I've had no problems with it. This is a reason that we love design like the Louverins, 311291s and 311284s. The bore riding noses of the latter two and the long bearing surface of the Louverins prevent this problem.

So, IMO, obturation is a key to accuracy in both rifles and revolvers (treating the autos as a rifle). As long as our pressure matches the alloy and we don't exceed the strength, we get good accuracy and minimal leading.

Just MHO./beagle

[S.R.Custom]

Personally, I try to avoid obturation if I can. In muzzle loading, it's pretty much a necessary way to do business, but it seems to me that if you have to rely upon it in a cartridge weapon, something is wrong and needs to be corrected.

The problem with obturation is that it happens so erratically. Variations in alloy from one bullet to the next will affect the extent to which it occurs. Variations in the powder charge and primer brisance will do the same thing by affecting internal cartridge pressures at ignition. Dimensional strangeness within the the cartridge case, chamber, throat, and bore will affect the symmetry of the obturation process. Internal anomolies/voids/whatever in the cast bullet itself will cause the same thing. And in any given weapon, all of these things could be occurring in any kind of proportional combination, insidiously conspiring to render a lovingly crafted projectile into a usless lump of crap..

For consistent accuracy, it's best to have a bullet hard enough to withstand the propellant charge behind it without deformation, and to propel said bullet through a weapon whose dimensional properties are correct.

[KYCaster]

Well, I'm kinda surprised at the number and quality of responses to my questions.

DD...."I can't but feel that there is limited benefit to be gained from obturation. Even then things are going to have to be near perfect to make it work. The bullet is going to have to be a close fit to start with. It's going to have start square to the line of bore and stay that way. Misalignment is going to be exaggerated by obturation."

That's exactly my reason for asking. "Logic" tells me that if you're depending on obturation to get you where you want to be then you have dimentional problems that should be addressed some other way, if possible.

BASS...."Take my Whelen with 2400 and RL15. 24 grains of 2400 is @ 27,000psi. That 27,000 psi peaks in .9" after the bullet starts moving. The same 27,000 psi peaks with RL15 after 1.99" of bullet travel. The obturating force on the base is higher with 2400 that only produces 1750 fps. While the pressure on the base with RL15 allows 1900 fps. So I can actually go up to 30,000+ psi before my ACWW hardness begins to collapse from too much pressure, so I now get 2100 fps."

OK, that's consistant with what you always say about pressure vs. hardness vs. accuracy and it makes perfect sense...but in the case of your RL15 load, by the time you reach peak pressure, the boolit is completely supported by the barrel so obturation is not going to occur at all. You'd have to increase pressure substantially before you'd see any change in the shape of the boolit, then I would think the nose would slump and unfilled lube grooves would collapse. Until you reach that point I don't see how there would be any effect at all.

Uhhh, OK that's what you said here....."So what kind of obturation, which will eventually turn into deformation you get, either in a handgun or rifle depends on how fast and how hard you spank that slug. If that pressure occurs when steel isn't supporting the lead, then you are going to obturate. If you obturate when it is supported by steal, either your lube has failed or you are compressing / deforming your bullet and you must either 1. cut pressure, 2. go to a harder slug, or increase pressure so that the collapse is most uniform."

Most of that makes sense. I don't see why lube enters into it, seems to me lube failure is a whole 'nother subject. Also "increase pressure so that the collapse is most uniform" sounds like that would be an extreme circumstance...kinda like swaging wad cutters inside your barrel.

Bass, I think you need to check your math here...."Often people stop looking for accuracy when the bullet first starts to deform when they may be successful going on up. This is how, along with a square groove bullet design, Elmer Keith was able to shoot an 11 BHN bullet at 34,000 psi when the 4122 X hardness chart says 11 BHN should crap at 22,000 psi. Handguners generally understand this better than rifle shooters because one confusing variable is left out ..... RPMs."

11 BHN X 4122 = 45342 which explains why ACWW can be pushed well over 2000 FPS. And 22000 / 4122 = 5.33 BHN, close to pure Pb. Now, this leads me to Beagle's comments, but this is getting pretty long, so I'll continue in another post.

Jerry

[KYCaster]

Beagle's comment...."Revolvers are different. When you light off a round, you get chamber obturation, with some blow by of propellant gasses. If it's of a correct hardness and of sufficient size, you'll get sealing in the chamber and I like revolver bullets soft and big to accomplish this. Once the bullet starts to exit the chamber and enter the forcing cone, another transation begins where the bullet either enters the forcing cone straight or missaligned. If straight, less deformation will occur and you'll get better accuracy. If missaligned, pressure will force the bullet to take the form of the rifling. While all of this is taking place, gas is escaping between the cylinder gap and also blowing by the bullet. If an imperfect fit, a small amount of lead is erroded and you get leading in front of the throat. Once the bullet takes the rifling, if the bullet is soft enough and large enough, almost complete obturation will occur and the gas is effectively sealed except for the cylinder gap. Some "skidding" will occur when the bullet takes the rifling. This will be minimized if the bullet is soft enough, big enough and the cylinder/forcing cone dimensions are within tolerance. It will occur to some extent because you're throwing a fast, soft projectile into slightly curved rifling."

OK, Beagle likes his revolver boolits "soft and big" to get the gas seal as the boolit makes the jump from cartridge to barrel. He sez, "If it's of a correct hardness and of sufficient size, you'll get sealing in the chamber..." Now if the 4122 X BHN is correct, a pure Pb slug which will deform at 22000psi should work fine in the mag. calibers, but you're really pushing the limits of the older Colts and S&W's. Others claim to like harder boolits in their revolvers and report very good results.

My point is, I think the "big" part of this equation is more important than the "soft" part. If you're depending on obturation to accomplish your goal, you have dimentional issues that could be handled just as well with a different approach. I'm not saying that one method is right and one is wrong. Sometimes one will work better than the other and sometimes neither will work.

One statement I think, needs some clarification... " Once the bullet starts to exit the chamber and enter the forcing cone... gas is escaping between the cylinder gap and also blowing by the bullet... if the bullet is soft enough and large enough, almost complete obturation will occur and the gas is effectively sealed except for the cylinder gap."

This phenomenon is almost universally accepted as fact, I've seen very little discussion that contradicts this. But, get out your calipers and I think you'll find a basic misconseption about this. Take the 38 SPL loaded with a 158 gr. SWC to an OAL of 1.465 or so. That leaves ~.4 in. of the bullet inside the case. Now drop the hammer and as the base of the bullet leaves the case mouth, the edge of the front driving band is leaving the cylinder. If your dimentions are right, the cylinder throat is sealed so there's no gas leaking here. Before the base of the bullet leaves the cylinder, the front band is well into the barrel and fully engaged by the rifleing. The driving bands, being wider than the cylinder gap, don't allow any gas to escape until the bullet's base clears the cylinder, so the only reason gas is blowing by the bullet is because of problems with the cylinder throat/forcing cone/barrel dimentions or timing problems.

These problems can be overcome to a certain extent by manipulating alloy hardness, bullet size and pressure, but correcting the cause of the problem, I think, is a better solution rather than depending on obturation.

Ricochet's comment about the pressure in a tight throated chamber is very interesting. I'd like to see more information about that, but, again, if your chamber dimentions are such that you have to resort to something like that, I think you're in a losing situation.

And I agree 100% with Supermag..."The problem with obturation is that it happens so erratically. Variations in alloy from one bullet to the next will affect the extent to which it occurs. Variations in the powder charge and primer brisance will do the same thing by affecting internal cartridge pressures at ignition. Dimensional strangeness within the the cartridge case, chamber, throat, and bore will affect the symmetry of the obturation process. Internal anomolies/voids/whatever in the cast bullet itself will cause the same thing. And in any given weapon, all of these things could be occurring in any kind of proportional combination, insidiously conspiring to render a lovingly crafted projectile into a usless lump of crap.."

"...insidiously conspiring to render a lovingly crafted projectile into a usless lump of crap.." In all the years I've been hanging out on this board I can't recall ever reading a more eloquent statement.

Thank you all for your replies.

Jerry

[mauser1959]

This is an interesting thread as I have looked at the alloys used by E Keith and come to the conclusion that they had to be in the range of around 9 , not the normal 11 stated. And one other thing , someone said that BP burned faster than unique ? I will admit to being confused but this one issue has bugged me more than most others have . Well that and rate of burn when building a new load ; which has me completely brain frozen.

[Lloyd Smale]

I allways get a headache with these "bumping up posts" Lots of guys like soft slugs to bump up but not I!! Ive got about as extensive of a collection of shooting records (for handguns) as anyone here. Ive tested just about every handgun caliber made and tested them with just about every alloy out there and have come to a couple conclusions myself. they may not fly with the technical guys but little holes in targets dont lie. I have never paid much attention to what the twist rate is in my handguns other then the fact that when shooting heavy bullets slower twists tend to do a tad better. Heres what years of sitting at the bench and measuring groups has taught me. IF YOUR GUN has proper critical dimentions. Throat forcing cone bore and grove and a good crown and good alignment youve just won 3/4s of the battle. If you dont your pissing in the wind. Sure you can cast soft and hope a bullet DEFORMS (true defintion of bumping up) to put a bandaid on your poor gun. But 95 percent of the time if everything is right with your gun you will get better accuracy with a harder bullet. I hear all the time about guys that claim they need to use soft lead to get accuracy but then in the same sentence will claim there gun does better with jacketed bullets. Makes no sense to me. If a bullet has to bump up to work jacketed handgun bullets wouldnt be worth squat to anyone as they wouldnt shoot in anything but the most perfect gun and ive shot enough of them to know that isnt the case. I hear also that to hard of a bullet causes leading at low pressure and again to that i say HOGWASH! It certainly will if your throats are two tight and you have blow by and your bullet is rattling down the bore but why the hell would anyone leave a gun like that when for 30 bucks a guy will open your throats up to the right size and for another 30 or 40 recut your forcing cone to work better with cast bullets. Now that ive said that ill also say that if your gun is right and it has a nice smooth barrel you can get away with some pretty suprising results even using pure lead without a bad leading problem but the problem you run into is BUMPING UP! Which like i said is DEFORMING. So the faster you push the bullet the more it deforms and the larger your group size will be. I guess alot of it is what a guy considers accurate. With enough load developement you can make about any bullet out of an alloy shoot well enough to hunt large game at 50 yards. But i will till i die shake my head at someone who tells me that they have to cast there bullets soft so they will bump up to fit there bores when for a few dollars they can fix there gun and it will shoot any alloy better. Most guys i tell this too tell me "sure you have all those custom guns and put way to much money into it and i cant afford it" but then you look and they have 3 or 4 handguns and as many rifles. Personaly id rather have one gun that was set up right and was consistantly accurate the 5 that i had to struggle with. A good example is my ppc gun. It was made by clark on a model 10. It has perfect dimentions and a glass smooth barrel. I shoot little 120 grain cast bullets cast out of lynotype and shoot them at about 800 fps max. Now you know theres about no pressure to speak of in a load like that. the bullet has a tiny lube grove so a guy cant claim that its the lube thats helping it a ton. I will clean that gun at the beggining of ppc season and shoot thousands of rounds through it in the year and clean it again at the end of season. Without so much as touching my barrel (i do hose off the rest of the gun occasionaly and oil it) at the end of season i have abosoulutely no leading in that barrel. As a matter of fact i dont know when the last time ive had to actually clean any handgun barrel because of leading and if you knew me personaly youd know that i shoot a ton of lead down range out of handguns every year. Why? probably because very few of them are out of the box guns. If something was wrong the gun was either fixed or it went down the road. I know very little about cast and rifles and allways have my ears open here to learn. The main thing ive picked up from the guys hear is that when shooting cast bullets fast like a rifle will the major problem is twist and rpms. Both seem easier to overcome again with harder lead. I cant believe that a rifle shooter wants his bullets deforming anymore then they have to either. Another thing i have picked up on and i thank bass for it is initial pressure with a cast bullet. I never gave much thought to trying to ease a bullet out of the case and having the pressure develope at the proper time in the barrel. I thank him because this has about changed all my thoughts on loading cast bullets not only in rifles but in handguns too.

[44man]

Very good discussion but several things don't jive. One is that obturation is needed in muzzle loaders :confused: NOT true. You will never get accuracy with a small ball or boolit. You need a larger ball or thicker patch. A boolit must engrave the rifling, even a Minie' will not shoot good unless it fits tight. There will be some obturation to fill the rifling better but to depend on it to expand a small boolit will always result in poor shooting.
Then everyone says black burns much faster, yes, maybe, but it does not reach the pressure levels of smokeless and once a certain pressure is reached it will continue to burn for the length of the barrel, not all at once. A heavy enough charge with a lot of powder will burn out in the air.
Yet black powder can slump a soft boolit nose so bad they go haywire so what happens to a soft boolit with 50,000# using fast smokeless? I picture it losing it's nose profile and shortening, losing the integrity designed into it. You might as well swage it out of whack first. In a revolver, how much lead tries to squeeze out of the cylinder gap? Remember the old soft factory wadcutters shot in .38's that would coat the front and sides of the cylinder with lead? Remember getting sprayed with lead? I blame poor fit and dead soft lead! How much chamber pressure did those old rounds have???
I agree with Lloyd on this. It is a mechanical problem and has to be fixed first. Once a boolit has the best support you can give it, boolit damage is reduced and accuracy increases. A boolit nose has to resist slump and might be the reason I get better accuracy from a truncated cone instead of a Keith boolit.
That is why I don't depend on obturation and don't believe in it even if every bullet/boolit does do it in different amounts. The important thing is to reduce it as much as possible.

[Pat I.]

Bass, I think you need to check your math here...."Often people stop looking for accuracy when the bullet first starts to deform when they may be successful going on up. This is how, along with a square groove bullet design, Elmer Keith was able to shoot an 11 BHN bullet at 34,000 psi when the 4122 X hardness chart says 11 BHN should crap at 22,000 psi. Handguners generally understand this better than rifle shooters because one confusing variable is left out ..... RPMs."

11 BHN X 4122 = 45342 which explains why ACWW can be pushed well over 2000 FPS. And 22000 / 4122 = 5.33 BHN, close to pure Pb. Now, this leads me to Beagle's comments, but this is getting pretty long, so I'll continue in another post.
Jerry

I believe the correct number is 1422 which would make the answer 15,600 psi.

My thoughts on obturation are that it's much ado about nothing. The idea of being able to control it in any way with a fixed cartridge would be an excercise in frustration so set things up right and forget about it..

[Bass Ackward]

Jerry,

Sorry. That was not the actual formula but the name of the formula.

This is the formula for making the Calculations: ALLOY: BHN X Tensile Strength (480) = Factor,
Factor X 3 = Minimum Chamber Pressure (psi), 4 X Factor = Maximum Chamber Pressure (psi)

WW 12BHN 12 X 480 = 5760, 5760 X 3 = 17,280 psi, 5760 X 4 = 22,040 psi

So minimum is 17,280 and max is supposed to be 22,041. So Elmer was clearly way over the chart limit.

And I get accuracy all over the place. Some lubes and loads it is just about when obturation should start. Other combinations it is closer to where the formula says it should end. Then I get a dead zone that kinda supports the chart and then I get better accuracy again. So clearly something else is going on.

And if your lube is changing the coefficient of friction, it's also increasing the time your base feels that pressure and can possibly raise that pressure. So poor quality lube lowers the figures you see listed above. So does a good lube if you didn't seal fast enough and blew half of it off. All this works hand in hand to explain why we all get results where we do. Now some guys are shooting LLA over 2000 fps for gosh sakes.

When it comes to cast, everyone develops a style based upon how they have achieved success. Their preferences for bullet diameter, lube, and hardness are going to be based upon success at that level. But there are ALWAYS other ways of doing things. What they have done to their satisfaction is learned how to solve the problems that they create with their methods. IF they choose bullet designs that are compatible with their methods, they will love them. Designs that perform differently will do something else and the bullet design is at fault. Then along will come another guy who has a different methods and makes the design they hate work like a champ.

BP was a perfect example. BP peaks immediately and the bullet deforms immediately. If you go back and look at some of the match results with BP and lead from the 18 century, we haven't come that far with rifled barrel accuracy. Accuracy is always easier with a perfect bullet and thus the chart was made above to show you where your perfect bullet would start to change. Weight is a factor to changing that chart. So is cartridge case shape towards how fast pressure comes up. So is lube quality. So is lube quantity if you tend to use methods that blow off more lube than the next guy. And so are many more factors to numerous to add.

But handgunners are a different lot when it comes to cast. Higher percentages of guys are always trying to push for more velocity out of lead. So they learn procedures to accomplish this better than rifle shooters do. And I will bet many here are above this chart deforming their perfect slugs and just don't know it. And if it works, who cares? Unless we are having this discussion. :grin:

The big picture is difficult to put it all together. And who says I got it right? All I know is that I shoot multiple styles and achieve .... fair results when I do it. I use lino for 38 Special loads and ACWW for 44 mag loads. I use WDWW for 1800 fps rifle loads and I use ACWW for stuff up to (currently) 2850 fps. I also shoot bullets here that would get me kicked off the board. I drill holes in them by hand. I drill in from the nose. I drill in from the side of the nose. As long as the base is good, I can make them shoot accurately out to the point where velocity slows enough that they can't be stabilized any longer. So velocity and RPMs are my friend, not my enemy.

The point of my answer to you was different things happen to our bullets under different circumstances based upon how we launch them. That is the purpose for this board. If one style could be nailed down and procedures established for one guaranteed method, posts would dry up fairly quick. Nobody knows exactly what is happening at any time. Only their theories and what steps they took to make theirs work. IF what they did worked, then there has to be some validity in their logic assuming they just weren't lucky. :grin:

[Bass Ackward]

When you use a fast powder the Time to deform is reduced. A slower powder will cause the process to act longer , therefore more deformation.

TT,

I disagree with this. If you ever are going to be hit from the rear, never sit still and appy the brake (fast powder), always hit the accelerator and try to increase the speed (slow) to minimize injury or damage.

Larry,

In Six Guns Elmer talks about why he choose square grooves over round. "He" believed that square was stronger to avoid deformation and collapsed more uniformly than round when it did. Does that mean round won't shoot? No. As long as you are using a hard enough bullet for your pressure and not collapsing it assuming Elmer was correct.

[twotrees]

Creep is a subject that folks get their PHd's researching and I was trying to put it in layman's terms here so it could be understood.

With lead (solders we have tested a LOT) if you apply pressure to it, it will try to move away from the pressure. It takes 3 things to make this happen. The correct amount of force, not so much as to distroy it instantaly but enought to start the reaction. Time there is an incubation period (stage 1 creep) steady change range (stage 2 creep). Then rapid tearing to failure (stage 3 creep).

The third fly in this ointment is heat. If heat is applied to a Pb alloy that will not melt it and force is applied that is less than will cause creep to start, at room temp, this process will start, over time.

So Stress+ heat/time is what causes the problem. The fast powder can produce the same stress (Pressure) as a slower one, but does so so rapidly that 1 part of the equation is reduced to the point of being below the critical threshold. Heat, a faster powder can reach the same temp as a slower one but over a shorter time therefore not enough specific heat to cause the reaction to start.

All of this is trying to look at a process that can be tried at home. Take a flat nosed boolit or 3 and put a hard (Steel) plate on the base and another on the nose. Add a diffrent weight to the nose of each (10lbs should do for a 44 cal boolit) Add more weight to each of the others. Let these sit for a week and measure them for lenght. they should be OK /same as they started. Take the 10# weighted boolit and heat it to 100 F while the weight is applied Give this 24 hrs and then measure the length. I'm making some guesses here on times and temps, but I'll bet there is a change.

If more force/more temp is applied the time becomes shorter, reduce either of the other 2 and time gets longer. (Slow powder vs fast powder).

All materials are subject to more than 1 limiting factor and this is just another piece in the puzzle of this simple game we play of casting boolits.

Good Shooting,

TwoTrees (Grew up in Uniontown, class of 64 UJSHS)

Mechanical Properties Research Lab
Ga Institute of Technology.

[Harry O]

Miscellaneous comments:

There are very big differences between rifles and handguns. However, through my own tests, I have found out that I get best accuracy in both by increasing the size of the bullet to the maximum that will easily fit in the chamber. That is usually much larger than the groove size (often .003" to .004" larger). Anymore, I don't worry about slugging the bore. I pay attention to the chamber neck.

My theory is that the bullet that is the largest that will fit in the chamber starts out straighter than a smaller one. The swaging down of that bullet in the bore/groove is less of a problem than starting out crooked. I do not know if that is the only reason, but I have run enough tests to know that it is more accurate than smaller bullets (but still larger than the groove diameter).

If you read Elmer Keith's books, he mentions several times that his loads leaded. He took that as part of the tradeoff for using lead bullet loads. Yes, he operated above the 1,422 x Bhn formula. It is not a hard and fast rule that makes bullets disintigrate when you step one psi over the line. You can operate above that limit if you know as much about lead bullets as he did.

A couple of things I learned about obturation when shooting 41LC bullets (both hollow-base and heel-base). The bullet obturates from the rear working forward. If you are doing it right, it does NOT obturate ONLY in the rear. That is inaccurate (even for a 41LC bullet). If it obturates full length (and if loaded correctly, it does), it is more accurate than any lesser load. Of course, no matter how a 41LC bullet is loaded, it will never be as accurate as a bullet that it correctly fitted to the chamber, groove, and bore. Obturation can help, but it can only correct small deficiencies (if done right), not major ones.

[Bass Ackward]

Interresting.

And I can buy that. I have always needed less lube and / or I can get higher velocities in the winter as long as I have a bullet design to handle the fouling.

Uniontown huh? OK. :grin:

[felix]

Right on, Harry. Not only for boolits, but for bullets. ... felix

[felix]

Twotrees, anything physical goes through the creep process. Taint' nothin' immune to that process. ... felix

[felix]

Don't worry about reducing it to as much as possible, 44man, just make obturation consistent from shot-to-shot. The BR boys clean the gun between relays in the hope for keeping things consistent enough to win the next relay. Keeping the neck area maintained is paramount, and that really becomes apparent using lubed boolits. ... felix

[Harry O]

P.S. I believe that "obturation" takes place with anything over zero psi. The problem is when that obturation crosses over from reversible to permanent. That is at the yield point and that point is 1,422 x Bhn.

Below the yield point (in the elastic -- or reversible -- range), you can get all the benefits of obturation (such as keeping the gas from following the rifling from the rear forward) without any of the disadvantages. Above that (in the plastic -- or pemanent -- range) you have to deal with the disadvantages (such as permanently deformed bases) because you will get them whether you want them or not.

Sometimes you can operate in the plastic range and have the advantages outweigh the disadvantages. That takes a good deal of experience, experimentation, and more than a little luck, however.

[felix]

Speaking of the creeping-crawlees, I've noticed that my electrical power bill has been going up gradually over the last several years. Making a long story short in terms of "lab" calculations, I had the power company come out and re-clamp my lines between my transformer and the trunk line to my house. Sure enough, the voltage difference was 15 volts on one of the legs comparing before and after re-clamping, and 5 volts on the other leg. You'd expect this when clamping two unlike metals, such as copper and aluminum over several years. So, the creepy-crawly will occur with like metals as well. ... felix

[beagle]

Harry O...Obturation starts from the rear but is also fell in a sideways manner if a space is available for access to the sides.

Being as you've shot the .41LCs from that HB mould you got from me, I know that you've noted this as well.

HB bullets depend on obturation to seal. You'll take a HB bullet with an imperfect cavity (maybe wrinkles) and after firing and recovering the fired bullet, you'll note that the cavity is elongated (deepened) and is perfectly round and uniform. This tells me that according to Pascall's law, pressure in a combined space is felt in all directions and Pascal was right.

This is a good thread for non-shooting weather./beagle

[Harry O]

Harry O...Obturation starts from the rear but is also fell in a sideways manner if a space is available for access to the sides.


Correct. I guess I did not point that out.

Starting out with low powder charges and increasing them (with your hollow-base bullet mould and 50:1 cast bullets), the rear of the bullet was increasingly deformed (meaning it permanently expanded outward and that portion was also shorter). With more powder, the amount of the bullet permanently deformed increased in length starting from the rear and working forward. With a full powder charge and soft enough bullets, it was expanded sideways amd shortened uniformly full length. The rifling grooves were uniform full length from the rear of the skirt to near the front of the bullet where the nose curved away from the sidewall. The bullet had to be expanded full length to get any improvement in accuracy over lesser loads (slower powders also reduced the amount of permanent deformation).

The resulting (recovered) bullet was shorter than it was originally and larger in diameter. Another thing that was interesting was that the final diameter was inbetween the original diameter and the groove diameter. The original diameter was 0.386". The groove diameter was 0.400". The final recovered diameter (after firing) varied from 0.390" to 0.394".

I believe that this also confirms my observation that part of the "obturation" is elastic and part of it is plastic. In this case, approx 0.006" to 0.010" (the part between the final bullet diameter and 0.400") was elastic and disappeared (or sprung back) after the bullet left the barrel. The part that was plastic (0.004" to 0.008") remained.

The most accurate heel-base bullets also permanently deformed at the rear. The most accurate ones expanded (and shortened) at the rear of the heel to grip the rifling at the rear of the bullet like the full diameter at the front also gripped the rifling.

What I have noticed is that you don't really need a hollow-base to "bump up" the diameter of the bullet. With soft enough lead, a heavy enough bullet, and fast enough powder, a flat-base bullet bumps up (or permanently obturates) just as well as a hollow-base. Not mentioned so far, the intertia from heavier bullets also increases the amount of obturation (I modified one of my Rapine hollow-base moulds to decrease the length of the base pin. I get 215gr bullets instead of 188gr bullets. Accuracy with the heavier bullet is somewhat better than the lighter.).

[w30wcf]

Interesting discussion. A small book could be written on the subject with the experiences of those here.

As we see, soft bullets will obturate at lower pressures which means that the bullet is "plastic" enough to put enough PRESSURE AGAINST THE SIDE OF THE BARREL to seal any gas from getting by, eliminating gas cutting which will produce leading.

Bullets in the higher BHN ranges that are even larger than groove diameter will not put enough pressure against the side of the barrel to seal the gases at lower pressures resulting in gas cutting and resultant leading.

w30wcf

[Harry O]

w30wcf: I think that is a pretty good summation. One minor detail: I believe the obturation that prevents gas cutting along the rifling is due to sideways pressure (caused by the obturation) against the side of the rifling (the 0.004" or so dimension). There is also an equal but outward pressure (caused by the same obturation) on both the circumference of the bore and the grooves, but that pressure is not what is causing the sealing against the rifling.

Just my personal opinion. BTW, engineers have a term for this shortening and sideways widening. Poisson's ratio.

[w30wcf]

Harry O,
Thank you. I wrote "bore" but I meant "Groove". Duh! I have since modified my post.

Not sure what you meant: "but that pressure is not what is causing the sealing against the rifling" in your paragraph: "There is also an equal but outward pressure (caused by the same obturation) on both the circumference of the bore and the grooves, but that pressure is not what is causing the sealing against the rifling."

I would think that the pressure against the inside of the barrel is corking up things pretty tightly when the pressure exceeeds the elasticity of the alloy.

w30wcf

[Harry O]

What I mean by that is:

The pressure caused by the gunpowder causes the bullet material to shorten and expand outward in all directions. That means there is sealing pressure between the outside of the bullet and the inside of the barrel both radially outward and perpendicular to that (where the sides of the rifling is).

The pressure of squeezing down an oversized bullet in an undersized bore (absent continuing pressure due to the gunpowder) would initially do that, too, but after the yielding happened, there would only be radially outward pressure, not the perpendicular pressure. And there would be MUCH less outward pressure than that caused by the gunpowder.

It would be somewhat easier to explain with diagrams. I will see if I can sketch and scan something -- no guarantees.

[44man]

Felix, very, very true and is one reason the boolit shape means so much to me. The wrong nose can slump off center different for every boolit. The Keith is not the ideal shape for an even slump. I have recovered fired boolits where the nose slumped to one side and showed rifling marks but the other side was clean. Neither is a long sharp nose on a rifle boolit.
As long as the nose shape can be controlled, then the least disturbance at the base by fitting the boolit perfectly will give the best accuracy.
An undersize boolit along with a nose shape that won't stay where it belongs is a losing battle. Each alone will not allow consistant obturation but combine both and you have double trouble.
If you take any given boolit and measure the Brinnel all over the nose, you can get different readings. Nothing in the casting process can ensure even hardness all over. The wrong nose shape or any unsupported portion of a boolit can make the lead flow towards the softer part.
A void in a boolit will bend the boolit in that direction as the air hole is compressed.
I have never believed a void in a boolit will remain when hit with the pressure. It is swaged to close the void and the boolit bends towards the void. Same result though, one side is heavier then the other.
This would be an interesting experiment. Introduce a void, shoot and recover the boolit without damage somehow and see if the void closed and what the boolit looks like.
Lead under that much pressure is like soft putty and must be restricted to maintain shape and the design must not allow the boolit to deform.

[runfiverun]

this obduration that 30-w is thinking of is exactly why keith wanted a square lube groove because as the base extended outwards and to the rear [moving bullet, remember this is how he pictured this] it squeezed lube out to the sides of the barell also,
kind of a coat as you go to put down a layer of lube under the lead to stop its sticking to the barell [ think pam]

[Harry O]

This is what I have been trying to explain.

Here are two sketches of two different things. The first one is what happens inside a barrel that is caused by a bullet that is obturated by the pressure of the gunpowder. All the pressure is equal, it is perpendicular to all of the surfaces inside the barrel, and all of it is there as long as there is pressure from the gunpowder. With most loads, that is all the way to the muzzle, although it is less there than at the chamber.

The second sketch is a little more complicated. It is what happens when you force a bullet into the rifling. To start with, the pressure is just like sketch no. 1. However, as soon as the rifling is engraved and the bullet is resized, it goes to radial pressure only and then pretty much drops to zero. This all happens within an inch or two from the chamber. Think of it this way. When you push a way oversized, hard-cast bullet into your lubrisizer, it is very difficult -- it takes a lot of force on the handle. However, if you push the SAME bullet into the SAME sizer again, it takes VERY LITTLE force. That is what happens to the oversized bullet when it is pushed into a smaller barrel. There needs to be a lot of force at the beginning, but afterward, it takes very little force to continue pushing it along. There is virtually nothing here to keep the continuing pressure from the gunpowder from forcing its way up the rifling between the bullet and the barrel (absent obturation).

Obviously, both things happen whenever a boolit is fired. However, the second one drops to zero (or close to it) very quickly. Only the first one continues on to the muzzle.

[felix]

Pressure tends to follow the CENTER of the projectile after movement begins. This attribute is taken advantage of in air conditioning systems even more by having air ducts rectangular in shape which gives much less air movment noise. Another reason why hollow base boolits work so well. ... felix

[whisler]

44man: I find your comments about uneven slump due to voids very interesting. In "The cap lock muzzleloading rifle" by Ned Roberts, he refers to projectiles being cast and then swaged (eliminating uneven slump due to voids?) followed by loading through a false muzzle (for perfect centering) resulting in excellent long range accuracy. Those old fellows seemed to know a thing or three, even lacking the theoretical knowledge. I'm very new to all this but it is fascinating.

[runfiverun]

harry-o
i think that if you dont have enough pressure to push out on the sides any more
you are at the maximum powder to bore ratio.
and since the pressure has diminished this far you should be near max velocity by this time also so even if the bullet [relaxes] at this point it already has the rifling engraved is crooked or straight and is on its way any residual push should be lineal in nature and any other problems arise from inconsistencies in the barrell , the muzzle crown,etc..

[Bass Ackward]

What amazes me from this and other threads is how many people shoot lead of a single style. If you don't want any leading I suppose, then you need to really adhere to the proper yield strength of the metal for the pressure.

But if you shoot any style long enough or fast enough, you are going to get leading eventually no matter what lube or hardness you use. Some styles, like snap shooting are going to get you there faster then almost anything else. I know there are some guys here that shoot handguns like handguns because you here comments every once in awhile that you can't tell if you have the right lube until you have 100 rounds or so through with that load. That's a heavy shooter there.

The old timers considered a lead wash normal as we accept a copper wash today. That's why they hailed GCs on handgun bullets when they first came out. Not to eliminate leading, but to control it from building up. But the copper cleaning information that we have been hammered with over the years and come to accept as gospel for complete and total removal, has been transferred over to our expectations for lead as well. And that complete removal tends to cause frustration. And I believe this is a disservice and disadvantageous for lead for those individuals that may either build a false expectation for leading or want to move on to higher volume sports.

Competitors that used to shoot long shot strings taught me to simply have a few low velocity lead slug with GCs and shoot a round. Then go right back to shooting if accuracy went south. They said why clean it out when you just have to shoot and put it back in there? They leaded up and shot until the lead built up on the face of the cylinders and then they were stuck and had to clean. Then they whined and moaned cause they knew what it was going to take to get back.

But loading hot and soft can and most often does lead even if not right away. If it builds up it's a problem, if it remains as a wash, it can be loaded around to achieve accuracy. But when it does, those hot loads can be and are for me, some of my most accurate loads out to any range that I shoot the other style. Now I am talking handguns here. I have no need for leading in a long bore that I can't see well enough to clean and bench accuracy most likely will be affected. But most folks don't seem to push for bench accuracy anyway.

Just strange how many people let leading dominate their technique all the time.

[Ricochet]

I don't get in a dither over a light gray lead wash, and do pretty much as you said above.

[Larry Gibson]

TT,

Larry,

In Six Guns Elmer talks about why he choose square grooves over round. "He" believed that square was stronger to avoid deformation and collapsed more uniformly than round when it did. Does that mean round won't shoot? No. As long as you are using a hard enough bullet for your pressure and not collapsing it assuming Elmer was correct.


Bass

Yes Elmer did discuss that but most often in many articles his preference for the square grooves was because they "carried the lube better" than round grooves. Elmer preferred "Banana Lube" and 1-16 alloy for his 429421 bullets. I've shot and recovered numerous of those bullets cast of 1-16 alloy and shot at 1390-1450 fps. Never had a callapsed lube groove, doubt Elmer ever did either, especially in his heavy .44 Special loads at 1200 fps.

However we mostly agree on the cause and effects of unwanted obturation, at least inside the barrel:-)

Larry Gibson

[Ricochet]

Banana lube?

[44man]

I have never worried about lube grooves. I have made molds and bought molds with every type and number of grooves. I just don't think it is that important. I don't believe lube is an important seal either. I think lube only keeps lead from building up too thick. Sometimes I shoot for months without a trace of lead, then sometimes I get small strips from the bore with the same loads. I just don't worry about it.
I still believe after boolit fit, that design is very important. I have picked up LBT boolits that look like they could be loaded again. I have a pile of Keith type molds but even though they shoot decent, 2" to 3" at 50 yd's, none has given me the accuracy of the truncated cone. The most accurate jacketed bullets I have ever shot are NOT semi wadcutters but truncated cones. The XTP is super accurate. I firmly believe that nose shape is the best for holding it's shape. Even a round nose can be better then a semi wadcutter. I don't like the way a Keith enters the forcing cone, the nose will not move the cylinder play to align the rest of the boolit. It depends on the little shoulder that can squish.
It was a great boolit with the old guns and the low pressure loads but has gone the way of the dodo with the super high pressure loads in modern revolvers.
A Keith with a long nose that engages the rifling before the shoulder hits the side of the forcing cone is best but every company thinks they know what a Keith should look like. But what happens if the nose slumps sideways in the throat?
The old 429421 was a good boolit and I shot thousands and thousands of them but never, ever shot less then 1", 50 yd groups with them. Most of the time a 2", 25 yd group was good.
I think Elmer is out of date unless you are shooting Colts or older guns that won't take pressure. Or light plinking loads.
Our guns have advanced and so should what we shoot. No sense having mechanical brakes on a BMW.

[montana_charlie]

I don't respond to this thread with any pretense of 'expertise' on the subject...just a preconceived notion of what obturation is, and how it can be helpful.

My first point (call it a 'belief') is...
A perfectly cast bullet, of the correct design, is the starting point for good performance - regardless of the type of firearm involved. Anything which modifies that bullet before it hits the target degrades it's perfection.

Second...
An undersized bullet which obturates to match bore size is an aid to accuracy, but a properly dimensioned bullet is a better answer - primarily because it gets modified less.

Third...
If obturation is going to be discussed as a means for matching a bullet to to the chamber/throat/bore environment in which it must perform, revolver information should be excluded.

To my mind, the bullet fired in a revolver must negotiate four separate environments on it's way to the muzzle. It is fired in a very short-barreled smoothbore (where it may obturate, depending on speed of the powder)...then it passes through a gap that leaves all of the bullet unsupported (but only small increments of it at a time) where it may be obturating into the gap...whereupon it slams into a smoothbore choke (forcing cone) that (perhaps) sizes it down while funneling it into...being engraved by (and asked to properly seal) the upcoming rifled bore.
This poor bullet may be 'obturated' and/or 're-sized' three different times before it is engraved by the rifling.

This activity is a technology unto itself...not really similar to the act of firing a cartridge in a rifle or auto handgun.

In those firearms where obturation can actually have a meaning, I agree with those who feel that using the largest bullet the throat will accept is the best choice.
If the bullet cannot obturate because it already fills the throat, it will only be 'reshaped' enough to fit the rifled bore while the obturating force is merely used to ensure a seal is maintained.

This is (as I see it) the least amount of modification possible...and results in a projectile which most closely matches it's original state when it reaches the target.
CM

[Marlin Junky]

Is there any chance obturation in a rifle could also be a function of randomly located air pockets contained in the cast boolits. I am a very meticulous caster but I'd be kidding myself to say there are absolutely no voids in my boolits because occasionally I'll weigh a boolit with a small void in the base and it will still be within a tenth of a grain of the average. I can only assume the voids are randomly located based on an imperfect pouring system... which BTW, ain't going to get an better. This is one of the main reasons I want to get into swaging.

MJ

P.S. If someone has already brought this up, I apologize for not reading the whole thread.

[felix]

MJ, Unless you get into high dollar full length gas checks, high dollar swaging dies and tools, you will not realize any accuracy gains by swaging. If you are into lever guns to the extent I think you are, I would entirely stay with cast projectiles. However, if you want something different to play with, I will encourage you to go ahead and embark. ... felix

[Marlin Junky]

MJ, Unless you get into high dollar full length gas checks, high dollar swaging dies and tools, you will not realize any accuracy gains by swaging. If you are into lever guns to the extent I think you are, I would entirely stay with cast projectiles. However, if you want something different to play with, I will encourage you to go ahead and embark. ... felix

Felix,

The swaging would not be for leverguns, rather for single shots and I'd be wrapping the projectiles in paper. Question: would the projectile then be referred to as a paper patched boolit? Isn't a boolit by definition a poured projectile?

MJ

[felix]

That would work, MJ!!!! ... felix

[AR10]

I have always cast boolits as hard as possible in everything I shoot. .25auto .38, 357, .41mag, .44mag, .45ACP, 3030, 30.06 and more I don`t remember. Almost no leading in
any of them. In the case of the.44mag no leading using GC.

[longbow]

I am coming in late here but figured that there are a few facts and a couple of opinions yet to be heard.

The first fact is:

Lead and lead alloys do not have a well defined yield strength as they creep at room temperature and the maximum tensile strength I have found is 12,350 PSI for an alloy of 8% antimony heat treated at 450 degrees F, quenched and aged one day. This information comes from a publication by the Consolidated Mining and Smelting Company Ltd. from 1955 (now Teck Cominco Ltd.).

So basically the majority of chamber pressures produced by smokeless powder and many black powder loads exceed the tensile/compressive strength of the hardest and strongest lead alloy by a significant amount. ACWW has a tensile/compressive strength of about 7000 PSI and pure lead about 2000 PSI.

The second fact is:

Boolits obturate or slump due to forces acting on the boolit resulting in stresses higher than the yield strength of the lead.

The third fact is:

The forces and resulting stresses in the boolit are highest at the base and lowest at the nose.

So to get this straight, from the first fact almost all loads you and I shoot develop pressures and stresses that exceed the the ultimate compressive stress of the lead Boolits we shoot. If the boolit was trapped so it couldn't move it would be swaged into a cylindrical slug with rifling on it.

However, the boolit is not trapped and can travel up the barrel so the pressure acting on its base produces a force that accelerates the boolit up the barrel. The force produces stress in the boolit - highest at the base as is it pushing the whole boolit and lowest at the nose because it is pushing air.

The fourth fact is:

Anywhere along the length of the boolit that induced stresses exceed the compressive stress of the lead it will obturate, slump, flow, fail.

What this means is that the base of the boolit will almost always see pressures/forces/stresses that will result in "obturation". How far up the length of the boolit these stresses exceed the strength of the lead depend on:

- the weight (length) of the boolit.
- the rate of acceleration (powder burn rate)
- the hardness/strength of the lead alloy

I have recovered cast boolits that have collapsed at the lube grooves - induced stress exceed the strength of the lead (smaller diameter at the lube grooves).

Bass posted the following formula which to my knowledge has been developed by people with far greater experience in loading and shooting cast boolits than I am ever likely to have:

"This is the formula for making the Calculations: ALLOY: BHN X Tensile Strength (480) = Factor, Factor X 3 = Minimum Chamber Pressure (psi), 4 X Factor = Maximum Chamber Pressure (psi)"

The number that comes from that formula is just what it says above - "chamber pressure". I see many posts that indicate the writer thinks this number is the required strength of the lead - not so.

There are several sources that claim as the formula states that the best performance of a cast boolit will be between 3 to 5 times the tensile/compressive strength of the lead. I have included info from "The Fouling Shot", C.E. Harris, the Los Angeles Silhouette Club and a website "www.frfrogspad.com" below.

Another thing that has to be taken into account is rotational acceleration and speed. As the boolit accelerates up the barrel so its RPM's accelerate also producing forces and stresses. If these stresses exceed the stength of the lead it
will fail. In short, imbalance (voids) or uncentered boolits are our enemy. Typical rotational speeds of cast boolits are in the 100,000 plus RPMS in rifles anyway. Any imbalance is bad as the forces and resultant stresses can be large.

So, on to opinions.

Theories and facts are fine but field results rule! There are certain things you can't avoid like the yield/ultimate strength of lead but there are also empirical factors such as boolit/barrel fit, barrel surface finish, lubricants, throat size., etc that can also have a significant effect.

I think the "guidelines" are good to know - a good starting point. For those that just want to load and shoot this may be all they need. For those that want to experiment and/or push the envelope they are still a good reference and starting point.

Most importantly all reloaders should be researching the correct starting loads for their cartridge and gun then carefully working up to maximum loads. Accuracy may be had a many load levels and most likely not at the highest safe pressure for a given gun with a cast boolit - or "J" bullet for that matter.

Longbow

More following:

"The Fouling Shot" Issue #81, Sep-Oct, 1989 "Match
Wheelgun and Load Preparation, page 81:

Correct bullet hardness for revolver target loads is about
8-12 BHN, depending upon the charge giving best bullet stability
and the chamber pressure generated.

The usable maximum chamber pressure of an alloy is a
function of its Brinell Hardness Number. As a rule of thumb,
optimum chamber pressure for adequate obturation without leading
is about four times yield strength. The conditions of firing in
a revolver are more severe than in a rifle, so this figure must
be taken as an absolute, though in a rifle this approximation can
be exceeded to about 5 times yield if everything is "perfect."


From C.E. Harris:

"The key in casting bullets is to tailor the alloy hardness to the chamber
pressure of the intended load. For most lead alloys the tensile strength in
psi is approximated by the Brinell Hardness Number times the constant 480. If
the chamber pressure is less than about 3 times tensile strength, you will
probably get leading because of inadequate bullet obturation. If chamber
pressure appreciably exceeds 4 times tensile your bullets will lack adequate
strength to resist the shear, compresssive and rotational forces in the bore."

And:

"Likewise, leading is likely to occur due to plastic deformation and failure
of boundary layer lubrication in loads in which peak
pressure exceeds about 4 times tensile strength.

Therefore, the range of optimum chamber pressure must be
related to alloy hardness. As in the following example:

Wheelweight alloy in the as-cast condition at 13 BHN:

13(480) x 3 = 18,720 optimum chamber pressure
13(480) x 4 = 24,960 maximum chamber pressure

For lower chamber pressures use a softer alloy, for high
chamber pressures you will need a harder one. Really
quite simple."

And from the Los Angleles Silhouette Club:

http://www.lasc.us/CastBulletNotes.htm

And from http://www.frfrogspad.com/miscellm.htm#Brinell

There is a formula that purports to relate Brinell hardness (BHN) to different operating cartridge pressures:

BHN x 1,422 = Pressure needed to "upset" or "obdurate" the bullet properly.

or

BHN = ( Cartridge Pressure / 1,422 )

The 1,422 number comes from converting the pressure in Kg/mm2 (which is what the BHN is measured in) to lb/in2 (which is what we use for cartridge pressure). That is: conversion factor = 25.40 (mm/in) x 25.40 (mm/in) x 2.2046 (lb/kg) = 1,422.

There are several sources that say the best accuracy for a plain based lead bullet is when the pressure of the cartridge is 90% to 100% of the strength of the bullet so the final formula would be

BHN = ( Cartridge Pressure * 1.1) / 1,422 or Cartridge Pressure = BHN * 1422 * .9

Veral Smith’s book Jacketed Performance with Cast Bullets and Richard Lee’s Modern Reloading 2nd Ed., cover this relationship great detail. The NRA's Cast Bullets book by Col. E.H. Harrison which is out of print and several of the Lyman casting books allude to this relationship by listing what bullet hardness is necessary for best accuracy at different cartridge velocities. They do not mathematically tie the hardness and pressure together, though, as the
first two books do. The above formula seems to hold true up to about 30,000 psi loads.

[Bass Ackward]

What this means is that the base of the boolit will almost always see pressures/forces/stresses that will result in "obturation". How far up the length of the boolit these stresses exceed the strength of the lead depend on:

- the weight (length) of the boolit.
- the rate of acceleration (powder burn rate)
- the hardness/strength of the lead alloy


Longbow,

Very good. But in addition to weight and length, you also need to add caliber.

And then you mention nothing of the bullet design. Some designs are better / stronger than others at resisting either chamber forces or rotational ones. And the final determining factor is the conditions of the mechanicals or the gun itself.


The minimum figures on that chart are why I keep it. The top end figures I develop using the criteria you list and my additions allow me to just blow it up. I can use 20-1 in places that it says lino is no good. So it is what it is.

[Bob Krack]

Lead and lead alloys do not have a well defined yield strength as they creep at room temperature and the maximum tensile strength I have found is 12,350 PSI for an alloy of 8% antimony heat treated at 450 degrees F, quenched and aged one day. This information comes from a publication by the Consolidated Mining and Smelting Company Ltd. from 1955 (now Teck Cominco Ltd.).

So basically the majority of chamber pressures produced by smokeless powder and many black powder loads exceed the tensile/compressive strength of the hardest and strongest lead alloy by a significant amount. ACWW has a tensile/compressive strength of about 7000 PSI and pure lead about 2000PSI.

longbow,

I have no intention of being destructively critical nor inflammatory.

Having been absent the casting/shooting scene for over 30 years, my observation is based on the meaning of some of the words you use and the interpretations of those meanings.

I do not see the relationship between "tensile" strength and "compressive" strength. Any relationship is purely coincidental.

Slump is a term accurately used to describe "plastic" deformation due to gravity or other like forces and the magnitude of the force will effect the amount of the resultant slump.

Obturate is often used to describe the effects of the plastic changes of the alloy causing the alloy to expand in diameter (depending on it's inertia vs compressive strength) as would be expected and desired as in "slump". Centrifugal (centripetal) force will also effect the diameter of the projectile (depending on it's mass vs tensile strength).

Obturate is also used to describe the unwanted deformation of the alloy because of inclusions of different massed material, unbalanced shape or mass, and who knows what else.

I have read nearly all of the" lasc" site information and most of the links long before I read your post. As a matter of fact that is the area I was most actively shooting in the early 70's and cast and loaded all my own ammunition.

I see the problems slightly differently than you (I think).

Certainly obturation (slump) is important and critical in the proper containment of the propellant gas. The alloy needs to be soft enough to "slump" and hard enough not to be sheared by the rifling nor unnecessarily deformed in shape to influence ballistic shape.

Gas checks can help by imparting some twist resulting in less shearing by the rifling and by a certain amount of cleaning (scraping and "de-soldering" the bore), but are no substitute for a proper alloy. The gas check can also act as a gasket to help seal the base of the projectile.

Your numbers seem to imply that a round ball of dead soft lead from a muzzle loader generating substantially more than 2,000 PSI would become a true cylinder rather than a ball? That Elmer Keith's 11BHN cast boolits with chamber pressures of 15,000 to 20,000 PSI (or more) would become cylinders rather than semi wad cutters? It just doesn't compute for me.

I guess I am mostly asking for a slightly more understandable statement of facts.

Vic

[felix]

Must calculate using these so-called equations with a time parameter to generate a factual statistic. Anything else is just bunk to me. ... felix

[sundog]

Felix, right on.

P = pressure

V = velocity

Hitting the base with P pressure when V = 0 is going to be different than hitting it with the same P when V > 0. All about where the pressure peaks. The longer it takes to reach pressure the faster the boolit is already moving.

[longbow]

VillageIdjit:

longbow,

I have no intention of being destructively critical nor inflammatory.

No offense taken. Responses in bold below.

Having been absent the casting/shooting scene for over 30 years, my observation is based on the meaning of some of the words you use and the interpretations of those meanings.

I do not see the relationship between "tensile" strength and "compressive" strength. Any relationship is purely coincidental.

For most ductile materials the tensile and compressive strengths are very near the same. I design using steel mostly and when I work beam bending stress there is tension in one surface and compression in the other. the same allowable stress is used for design in both surfaces

I figured this might cause a bit of confusion if I used the term "tensile stress" then referred to compressive failure.

If I were designing a swaging die for lead I would use the tensile stress to determine how much force I need for a ram of a given diameter to force the lead to yield and extrude. This would tell me how much leverage I would need for a manual swage or how large and what pressure a hydraulic cylinder would have to be for a hydraulic ram.

Your numbers seem to imply that a round ball of dead soft lead from a muzzle loader generating substantially more than 2,000 PSI would become a true cylinder rather than a ball? That Elmer Keith's 11BHN cast boolits with chamber pressures of 15,000 to 20,000 PSI (or more) would become cylinders rather than semi wad cutters? It just doesn't compute for me.

If the round ball or a bullet were contained in a blind die they would indeed become rifled cylinders because normal charges of powder develop pressures exceeding the strength of the lead. Because the nose of the boolit (or side of the ball facing the muzzle) has no load on it - nothing restraining it - there is not enough inertia to cause it to deform. Rotational forces are another issue.

I'm not sure if this is a good analogy but it is the best I can think of right now. Think of a tall office building and how much load is on the roof compared to how much load is on the foundation. Maybe a rocket is a better analogy - the nose of the rocket is pushing only air but the rocket motor is lifting the entire weight and accelerating the mass of the rocket during lift off. The structure of the rocket would fail if it could not resist the force of the thrust.

I guess I am mostly asking for a slightly more understandable statement of facts.

Vic

I hope that helps.

Bass:

Very good. But in addition to weight and length, you also need to add caliber.

Caliber doesn't affect the stresses in the boolit at a given pressure.

Rotational forces will be higher for a larger diameter (caliber) boolit at the same RPM's.


And then you mention nothing of the bullet design. Some designs are better / stronger than others at resisting either chamber forces or rotational ones. And the final determining factor is the conditions of the mechanicals or the gun itself.

Where boolit design comes in is:

- area of contact with rifling to resist shear
- fit of boolit to barrel
- length of unsupported body (say long tapered nose) subject to slump and/or rotational forces
- area at the bottoms of lube grooves: I have had boolits collapse in the lube grooves with ACWW and high pressure loads.
- gas check or plain base
- I'm sure there are more too

I certainly agree with your comment about the conditions of "the mechanicals or gun itself" and commented in my post - unless you are meaning something else:

Theories and facts are fine but field results rule! There are certain things you can't avoid like the yield/ultimate strength of lead but there are also empirical factors such as boolit/barrel fit, barrel surface finish, lubricants, throat size., etc that can also have a significant effect.


The minimum figures on that chart are why I keep it. The top end figures I develop using the criteria you list and my additions allow me to just blow it up. I can use 20-1 in places that it says lino is no good. So it is what it is.

This would be the field results that seem to disprove some theories. You have developed loads that work and provide better accuracy and/or higher velocity than conventional wisdom says can happen. You have mated the right boolit design, fit, lube, etc. to the gun and worked up an appropriate powder charge. You have probably tuned the gun and maybe even lapped the barrel.

Something else I haven't gotten into here either is the issue of powder burn rates which is another discussion on its own.

Anyway, this is getting long so I better sign off.

Longbow

[Harry O]

And from http://www.frfrogspad.com/miscellm.htm#Brinell


Does anybody know who this "frfrogspad" is? He has lifted a large part of an article of MINE and has not attributed it as far as I can see. What you are quoting here is what I wrote.

BTW, did you happen to notice the 3 x 480 number is 1,440. Sounds awfully close to 1,422 doesn't it. Different people are finding the same thing even if they are coming at it from different directions. I have said many times that 1,422 is not a hard and fast "wall" that causes the bullet it disintegrate when you go one-psi over it. Things just start to get worse until the tricks that people use around here don't work anymore. Saying you have to stay between 3 x 480 and 4 x 480 is just another way of saying that.

Also, there is nothing I have been able to find in the literature on a lead/tin/antimony mixtures yield/ultimate strength of the type we use for bullet casting. The reason is that the only data I have been able to find is for mixtures used for soldering. There is a reason for that. Soldering is something that people do for money. The stuff that is soldered is often expensive (such as the computer we are typing on. It is worth money to manufacturing people to have hard data on soldering, so the industry organizations they support will do research in that direction.

That leads to just one problem with the information that you turned up (12,350psi in some industry manual) is that it is probably for SHEAR strength. SHEAR strength is what all soldering is based on. Yield shear strength varies a lot from material to material, but it is USUALLY approx 60% to 2/3 of the yield strength. I have read a LOT of industry literature and ALL of it is based on shear strength, not yield or ultimate strength even though some of it is careless on specifying that. They very often just use the word "strength".

[Harry O]

longbow,

I do not see the relationship between "tensile" strength and "compressive" strength. Any relationship is purely coincidental.

Vic

I also disagree with this statement. Any engineering book you want to look at stays that (at least with metals) tensile strength and compressive strength are very close to each other.

In addition, those same books say that hardness tests (such as Bhn) are closely related to tensile strength. If you know one, you can predict the other. Therefore, if you know the hardness, you have a close approximation of the compressive strength of that metal. In this case, a bullet (or boolit). That is what this is all based on.

[Harry O]

I would be VERY interested in seeing the entire publication. If you are interested in doing so, send me a PM.

Like I said, I have seen a lot of publications that are very loose in their terminology. They quite often do not specify what kind of strength they are listing. Only by crossreferencing with other publications can what it really means be tracked down (and only sometimes, even then). Interesting that the Bhn of the lead is listed here as 4.2. That is what I have run across several times in the books for extremely pure lead. Never seen it in real life, though. Even the so-called "pure" plumbers lead checks out at about 5.0.

I used to be employed by a metal manufacturing factory. As the head of the testing lab for 4 years, I had access to some VERY large equipment for Bhn testing -- and the equipment was checked for accuracy on a regular basis. When I was learning, I did a lot of testing (and some of it was on purchased lead bullets) to learn what it really meant. Unfortunately, that was before I started casting my own bullets. After starting casting, I did not access to that equipment, but I have had several small Bhn testers and currently use the Lee.

[Bob Krack]

I also disagree with this statement. Any engineering book you want to look at stays that (at least with metals) tensile strength and compressive strength are very close to each other.

In addition, those same books say that hardness tests (such as Bhn) are closely related to tensile strength. If you know one, you can predict the other. Therefore, if you know the hardness, you have a close approximation of the compressive strength of that metal. In this case, a bullet (or boolit). That is what this is all based on.

Thank you, Harry.

As with I did not with longbow and do not want to try to start a argument, I will try to research the concept of the basic similarity between compressive and tensile strength in metals. You are both probably correct and I suppose I reacted with a knee jerk (or maybe just a dumb jerk) when I questioned the statement.

Thanks for the replies,

Vic

[longbow]

VillageIdjit:

As mentioned, the tensile and compressive strength of mild carbon steel for both yield and ultimate strengths are considered the same. Also true for most other ducltile metals/materials. I have designed lifting devices for lead anodes and the lead lifting lugs on the anodes were designed using the ultimate tensile strength of the lead. When the lug is loaded one side goes into tension and one side goes into compression and the lug is in shear as well.

Brittle materials are another matter. Take concrete for instance. It has very high compressive strength but almost no tensile strength at all. This is why steel rebar is put into the tension sides of concrete beams, or why pre or post tensioned concrete slabs are used - the tensioning rods or cables apply a large compressive stress to the slab cancelling the applied tensile load (well, at least maintaining compressive stress under load).

I have a copy of the publication I mentioned and will check on copyright issues which I doubt will be a problem. If I am allowed (probably) I will scan and PDF the booklet and send it to the site for access by all. There is a lot of good info in it though most of the best is in the tables I posted.

Longbow

[longbow]

Unfortunately I have had to remove a previous post due to posting information that may be considered intellectual property.

I have been told I can read and repeat the information as long as the source is stated (which it was anyway) but I cannot copy or scan directly.

If anyone has any questions on the physical properties of lead and lead alloys - strength, hardness, heat treating, etc. I would be happy to look the info up and provide the data and source.

My apologies.

Longbow

[Harry O]

One point of clarification. I was once asked, "Why bother using hardness (Bhn) to predict tensile (or compressive) strength of metal instead of just testing what you want directly?"

The reason is because hardness tests are quick, easy, and NON-destructive. A direct tensile or compressive strength test is destructive.

If you want to test what the strength of something that is already built (such as a bridge), you can test the hardness anywhere you are suspicious without otherwise harming the structure. If you want to test the tensile or compressive strength directly, you have to cut out a piece of the structure, machine it to a specific size and shape, then destroy it. That does not do the remaining structure any good.

I used to have a portable hardness tester that I could throw in a briefcase and carry with me to wherever in the entire country the problem was. I often used hardness tests to narrow down where the problem was, then had a section cut out (with repairs done to compensate for the hole) and had it tested to destruction.

Hardness tests are used basically for two things. Used directly, they are used to predict life under friction loads (wear). Used indirectly, they can be used to predict tensile or compressive strength. That is what we are doing here.

[BAGTIC]

"I have been led to believe that this obturation takes place not in the case mouth but in the throat when the bullet hits the rifling and meets resistance. Prior to that the lead bullet is just so much ejecta. Well maybe not that simple, but you get the idea."

I recall seeing a couple of articles over the years with photos showing that 'obturation' occurs prior to the bullet entering the throat or engaging the rifling. One was a .45 Colt revolver that had the barrel removed. The bullet was notably both foreshortened and expanded. If this happens with a low pressure cartridge like the .45 Colt it surely must happen with high pressure loads.

[405]

Bagtic- agree,
Likely so since the first primary resistance to forward movement for the bullet from dead stop in the case to its acceleration down the bore is the inertia of the bullet not the friction with the bore.

[BAGTIC]

"In Six Guns Elmer talks about why he choose square grooves over round. "He" believed that square was stronger to avoid deformation and collapsed more uniformly than round when it did. Does that mean round won't shoot? No. As long as you are using a hard enough bullet for your pressure and not collapsing it assuming Elmer was correct."

Doesn't make sense. A round groove is just a square groove with a round fillet radiusing the corner. In what other field of mechanics are we asked to believe that a right angle relief is stronger than a rdiused one?

[405]

Bagtic,
The only thing I can think of about the shape of the ridge between grooves is that the bearing surface is larger on a square top ridge as opposed to the smaller surface of the radiused ridge top. An example might be: Take a pure lead rectanglular block and a pure lead triangular block. Put equal pressure on both and measure which deforms more..... Shoot.... I don't know but it's the only thing I can think of :???:

Also, if the mold is designed for squared ridges and radiused ridged bullets pop out then there is going to be some inconsistancy in weights and radial balance with that bullet and among bullets so cast???

[runfiverun]

keith didnt want the square lube grooves for boolit strendth
he wanted it to hold the lube in the groove better
which i believe it does
I like the back half of elmers boolits but....
i actually prefer a good rnflaatpoint for all around use rifle rev. pistol, lever... etc
seems best to me

jmho