Home Forums Hunting & Shooting Ask The Gunwriters Can "Bolt-Thrust" be scientifically measured?

Dr. Oehler,

I have done some study and research since I last posted on this subject of bolt thrust. I have found as many "experts" who agree with my synopsis as disagree, but after thinking this through I will have to concede that I am wrong.

Your statement about brass being bubble gun got me thinkin' and that hurt, but I realize that's what goes on under the pressure of combustion. A brass case cannot contain the pressures generated and though the behavior of brass will vary due to case shape and other factor, it cannot diminish the rearward thrust against the bolt. The malleable brass may act as a gasket to prevent the leak of gas, but won't reduce the movement of the casehead as the brass will stretch against the bolt.

Now I do think the straight sided cases will behave differently under this pressure, such as the movement or flow will vary, but you're right, it won't reduce bolt thrust.

When firing, the case sides will be firmly held against the sidewall of the chamber, but no matter how strongly held, the brass head will move rearward to consume any excess headspace, by stretching, if the sides don't give, and the full force of the pi r squared*psi will be felt against the bolt. Well, minus a "negligible" amount used to stretch the brass. I'm almost always this hardheaded but not usually this far out in left field. I'm not easily led astray, so I'll say I came up with this on my own, I mean being wrong. I have read several things that would lead me in the wrong direction but I'll take credit for this detour.

About pressure and the brass expansion, I have no engineering document but have watched as velocity and case expansion increased together to a point where expansion increased more than velocity and have drawn an association between them. That varies so much from different brands of brass and anealling, etc. so it isn't sound, but within certain limits it is one of the indicaters. Also, when chronographing I watch fps per grain of powder of powder and when the velocity increase drops, I consider that the knee of the curve and back off. I can do the math but I don't claim to be scientific. I have loaded and fired and chronographed a big bunch of ammo, much of it experimental with no data support from a cookbook, and never damaged a gun, or even come close, as far as I can tell.

Thanks very much for your expert tutelage and for all you have done for this industry.

Murphy,
As a student of these things, I'd love to know what research and musings of others you've run across that has swayed you to believe that brass behaves completely as a plastic? Ackley and Vaughn's experiments remain the most compelling bits of evidence that I've run across.

Dr Oehler,
I think you'll like this article;
http://www.shootingsoftware.com/ftp/dbramwell%20july%2019%2004.pdf Denton Bramwell beat me to it and did it in better fashion that I would have.

This is a complex subject that resists simplification.

Another factor to consider is that the case walls will carry more of the breach thrust in a smaller diameter cartridge than they will in a larger diameter cartridge. The reason being that the area of the case head increases by the square of the radius whereas the circumference of the case head increases by pi which is a linear function. Neglecting case wall thickness which doesn't vary that much, this means that there is proportionally more brass to resist casehead thrust in a 223 than there is in a 7mm Mag.

David White is the gunsmith who ran the tests on the breech thrust of standard cartridges versus AI cartridges. He rigged up Contender barrels with an open breech and flapper device to set off the primer. He did show that straight wall cases would stay in the chamber where tapered cases shot out the back. It didn't work with the 45-70 however. The 45-70 does not have much taper but the case still shot out the back of the barrel. Another detail which makes me hesitate to generalize from his experiments is that he used Contender barrels which were probably no more than 14" in length. What would have happened if he had used rifle length barrels of 22" or more that would have a longer pressure curve?

Ackley's experiment with the 30-30 is often cited in these discussions. People tend to overlook the fact that Ackley fired factory 30-30 cartridges in an AI chamber. So he fired a thinwalled, small diameter 38,000 CUP cartridge in an oversized chamber which would lower maximum pressure even more. I don't know how this is supposed to apply to a normal 52,000 CUP cartridge.

Interesting subject but I don't trust caseheads to hold pressure!

I don't know why any shooter is concerned with "bolt thrust", but firearms engineers actually measure it in test firearms, using instrumentation on the test bolt. You cannot do that with your rifle, and it would be pointless.

I too also wondered why nobody brought up the P.O. Ackley tests before now.

The big question originally on this thread: Do "improved" cases produce less bolt-thrust, and thus are somehow more efficient or special or whatever?

My main observation is that in any cartridge producing pressures above the average .30-30, a brass case isn't strong enough to contain the pressures without stretching. If headspace is excessive enough, the case will come apart.

I base this on have encountered a lot of rifles with excessive headspace, either in the rifle itself or created by dumb-ass case sizing. In cartridges like the .30-30 the primer tends to back out--at least if the chamber and case are dry, not lubricated somehow.

In cases that produce substantially more pressure, the primer does back out first, but then the case stretches inside the chamber--and back over the primer--producing that classic illusion of excessive pressure, a primer flattened by too much headspace rather than too much pressure.

I have seen this in cartridges of all sorts, including "improved" rounds or "modern" designs that feature steep shoulders and near parallel case walls. My conclusion has thus been that AI'ing does not "strengthen" a brass case enough to make any difference, because a brass case isn't enough to hold 60,000 psi anyway, no matter what the shoulder angle or how nearly parallel the case walls.

John Barsness

Chris,

Did I say plastic anywhere in my post. I said Ken called it bubble gum. I called it maellable. What are you getting at. Do you have an opinion of your own?

Murphy

Murphy,
You did not say "plastic", but what Dr Oehler describes is also known as a "Plastic Deformation". "Malleable" as you put it would be another way to describe it. I'm sorry for changing words on you. That's baggage from my other life.
scienceworld.wolfram.com/physics/PlasticDeformation.html
http://en.wikipedia.org/wiki/Deformation

What am I getting at? Nothing. I'm trying to keep an open mind and look at whatever evidence is out there. Do I have an opinion of my own? Yes, as a matter of fact, I do. It's still formative and as I said earlier, Vaughn's work and Ackley's work are carrying the most weight. Marc's description of Dave White's work is interesting, but I'd like to see the text rather than take it on hearsay. My question to you is an honest request to understand what you're seeing and has swayed you so I can add that to my own understanding. Nothing more.

What do I believe? I believe that case/chamber wall adhesion does reduce the backthrust of a case. I don't believe brass cases behave completely as a "plastic" (the physics term, not the petrochemical/polymer context) I believe the Ackley modification does reduce bolt thrust when compared with parent cases that are more severely tapered. I believe that in actions with more "flex", Ackley-izing can reduce the signs of pressure on a case and related to that, folks were able to show better vels without previously seen pressure signs on cases. I believe an Ackley case can provide more velocity at the same pressure, but it's likely to be modest and related to the gain in powder capacity. I believe a substantial part of the gains claimed for Ackley cases are from the case design allowing greater pressures without it showing negatively on the cases.

ChrisF,
Thanks for pointing me to the article by Bramwell. I ran across it a couple of months ago, and pretty much agree with what he said. I paraphrase loosely, "Primer appearance and case head expansion do correlate with pressure, but not close enough to call them a measurement. "
KenO

You summarize it well. My referring to the brass as bubble gum was obviously not technically correct, but it did serve to move the discussion to the point were case shape did not dominate the prediction of bolt thrust.

I hope that Murphy will agree that the correct answer probably lies somewhere between his original position and the position I took at the other end. The problem is that there are so many other variables we can't measure and we can't really predict what the actual bolt thrust will be on a round-to-round basis. All we can do is to design for worst case or else we trust the angels to take care of us.

Murphy,
Please don't flog yourself. We seem to be having one of those "number of angels dancing on a pin" discussions without definite answers. I'm not even sure of all my answers, but was bouncing the ball to stay in the game. You are a true gentleman, just like most of the other shooters I've associated with, and I do appreciate your kind comments. If all posts were as thoughtful and civil as yours, the discussions could be much more pleasant.

KenO

If somebody with a little engineering background was so disposed they could calculate how much pressure a brass case can contain before it fails and separates. Brass, like any material, has a "yield point" at which it will behave like plastic or bubble gum. That is, once it begins to fail it fails very rapidly. It should be easy enough to find the physical properties of cartridge brass and from there it is a simple matter of measuring case wall thickness and base diameter to find the pressure at which any given case will fail.

Anecdotal evidence indicates a 30-30 case has enough brass to contain 30-30 pressures. Anecdotal evidence indicates that the .470" and larger diameter cartridges like the 30-06, 7Mag, etc. don't contain enough brass to contain 30-06 or higher pressures.

OK shooter, and a couple of others have performed a finite element analysis with similar results. A dry case will stay forward until a pressure around 40 kpsi. That is why primers will protrude with headspace in low pressure cartridges. Oil the case and it comes back earlier, no protrusion, no riveting.
What is of interest is that once the case comes back it comes back with the same force oiled or not. This means that the case comes back during peak pressure, and that the brass when it yields will contain little force.
Good Luck!

I found this photo of a sectioned 30-06 case (happens to be a flare round) the area that stretches/seperates is back where the powder is (left side). Would one of the Ackley fans please explain how blowing out the shoulder adds metal, and there by strength, to this (wall/web) area?
.
I will grant that AI users tend to be experanced reloaders, and that they keep the clearance between the bolt face and the case head to a minimum, thereby reducing the available room for stretching to occure (minimize the clearance with a standard round and you acomplish the same thing).

Chris,

Sorry, didn't mean to be so obstinate.

Well, I too believe that that Ackley (or other similar improved) cases will yielld more velocity at the same pressure. I don't think there is a number (fps) that can be given but within a given set of cricumstances, etc. there will be a gain. I shoot several Ackley calibers.

I have believed for a long time that the straight sided case of the AI reduced bolt thrust. They do tend to stick to the chamber walls very tight while under pressure, but what I get from Ken Oehler's comment about the "plastic" is that even if welded to the side walls, at 60,000 psi, the rear portion of the case will move against the bolt as if it were "bubble gum" I think that was his wording. Now to stretch the case will reduce the bolt thrust but by such a small degree as to be insignificant. I think Ken's word was "negligible". I think I'll subscribe to that.

It would make sense that if the case sticks tighter to the chamber walls, that at the least it would "delay" the case setting back against the bolt head (until pressure is enough to stretch it) and that could change the case head signature.

I agree with your last statement about the case shape masking pressure signs and I have stated that several times. I was asked to describe that or quantify it and I'm working on that.

One of the things that changes the way the case head looks in the AI vs the std is that, when correctly chambered, that is at zero (or very close) headspace to allow firing factory minimum SAAMI spec new brass in the chamber of an AI, this "new" minimum headspace changes the look of the fired case from the way it will look with some few thousands of space. Another point is that even when excess pressure is reached, the cases fall out of the chamber . The do not cause a sticky bolt lift as a standard case would likely do. This is probably the biggest issue as most folks don't take measurements of the brass and study the case head to see subtle indicators of excess pressure, and don't notice the HP signs until the primers fall out..

I think the headspace dimension of the standard 30-06 case is 2.049"-2.055" from the head to the datum point on the shoulder of the case. This datum point is further defined as the point of diameter on the shoulder where it is .375", about midway on the shoulder. When I have new barrels made for the AI calibers, (or for any caliber)I send new brass of the kind I will be using (Lapua) and ask for this to fit with zero clearance on bolt closure. Or, specifically ask for minimum 2.049" (or what ever for the caliber) chamber dimension. This is not the same as rechambering a standared caliber to AI. When that is done with out setting the barrel back, it often has excess headspace. Mostly AI chamberings are re-chambering the existant barrel without setting the barrel back and there is a "gap" in the chamber that the case expands into when firing.

I digress. So back to the plastic case.

What I find most interesting is your comment about action flexing. I think that has merit. I know action flex is and issue in several ways.

Ackley is still interesting and spawning debate, after all these years.