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I keep hearing all these myths about bolt thrust on other forums until I am about to believe some of it.
If chamber pressure is 60,000 PSI then that is 60K in all directions, so how can we increase/decrease the pressure on the bolt by changing the shape of the chamber? For example a straight walled case is said to apply less pressure to the bolt than a tapered wall case.

Is there something here that I am missing?

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No, you're not missing anything.

There's a way to actually measure bolt-thrust, and I'm gong to publish an article about it within the next year.

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Thanks, John

I will be watching for you article.

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My undestanding is that it is an issue with lever guns as the failure point could be the rear lockup of the bolt as opposed to the cylinder integrity. The backforce on the bolt is the pressure of the cartridge times the diameter off the cartridge.

So a 30-30 at a given pressure will generate less backforce than a 356 Win at the same pressure as it is a skinnier cartridge. Don't think there's any debate on this point.

The AI folks say that the shape of the AI cartridge, being more straight than the original, will grip the chamber walls more tightly and transmit less force back to the bolt. So you can load the AI's to a higher pressure and still be safe as the bolt thrust will be mitigated somewhat.

I gave up on the 30-30AI years ago, so I don't really care whether the claims are real or imagined. JB has often pointed out that the velocity gains with the AI cartridges are due to them being loaded to higher pressures, rather than from the miniscule amount of case capacity increase from the AI. But the whole idea has filled many internet threads.

It is a function of chamber pressure, and the inside rear surface of ones cartridge. Sort of the same reason we have small pistons in high pressure hydraulic pumps.

Looking forward to Mule Deer's article, that will be interesting...curse of being an enginerd.

We use the concept that 1minute outlined on a daily basis in my field..the thrust load is simply the pressure multiplied by the normal area it acts on (very idealized approach). This gives us a ballpark number to roll with.

Once you get the axial bolt load, then you can start looking at lug shear loading and all that fun stuff...gives you an idea how bolt guns operate with respect to "safety" margin.

With rifle cartridge a little more involved than most people assume. Will know more after some actual measurements of case-head presssure on the bolt face.

Yup, most definitely more complex than the idealized case. I've messed with some pressure/compressive sensitive film between the bolt face and cartridge that I "calibrated" between hardened platens in a material test frame. It was in the ball park but more going on than simply P*A. Looking forward to your article.

There's definitely more involved in bolt thrust than chamber pressure and inner area of the case head. I have fired 30-30 rounds that did not touch the bolt face at all as evidenced by the primer protruding from the pocket. It has been said that P.O. Ackley once fired a 30-30 rifle with the bolt completely removed from the rifle and the case did not leave the chamber. I have some older 35 Rem cartridges that have crimped primers to keep them inside the case, presumably because there was not enough rearward case movement to reseat the primers upon firing (case head never touches the bolt face). I definitely don't know all the answers, but i do know there is more to it than meets the average eye.

I played with some wildcat cartridges in a model 94 Winchester a few years ago. I don't have pressure testing equipment but it looks like the brass case will contain the pressure at 30-30 pressures. A little over 40,000 PSI the case starts to stretch. Once it starts to stretch it appears that the load is very rapidly transferred to the bolt.

I also suspect that time is a factor. A smaller case using a faster powder will stretch cases less than a larger case using slow burning powder.

I have trouble believing that AI cases reduce bolt thrust. It is not difficult to make an unimproved 30-06 or the more tapered 6mm Remington case separate which tells me the case is gripping the chamber walls very nicely. What a straighter case wall does is a mystery to me. Maybe Mule Deer has the answers.

Don't have any answers yet, and doubt I will. All I usually do it report on what happened, and let the reader come to their own conclusions.

Getting the case lube off completely makes a big difference, but that probably goes without saying.

Actually, that is very much worth saying. Lubricated cases can indeed increase bolt thrust over non-lubricated cases. I have lubricated cases on purpose at times when fireforming.

Some time spent with Dan Lilja's work on bolt lug strength is worthwhile, as bolt thrust is part of that scenario. This link takes you to that article.

Good shootin'. -Al

http://www.riflebarrels.com/articles/custom_actions/bolt_lug_strength.htm

The case will push forward from the primer. In a dry chamber
it will stay forward until somthing like 40 KPSI. That is why
lower pressure cartridges with head space will have protruding
primers. About 40 KPSI the front of the case will stick and the
case head will stretch back leaving a ring. If the primer was
protruding it will be pushed back flattening it. If an oiled
case is fired it comes back and stretches forward avoiding
the stretch ring. In both cases the oiled or dry a higher
pressure case will have the same "thrust".
Even if you know the thrust you don't know the stress flow
so you can't calculate "safety factor".

I have attempted to measure variations in bolt thrust with a crude crusher set-up on an old Lee Enfield. While it seemed possible to compare various loads and while the measurements of compression/stretch were predictable, I had no real way to quantify the amount of thrust. In other words, I could tell when thrust increased but could not really tell what pressures were.
Variations occured with changes in brass manufacturer. Light loads with a lubricated case showed greater back thrust than did the same load in a dry case. Once pressure was increased to the point that cases were stretching and the bolt was compressing significantly, there was no real difference in lubed vs dry. Once the pressure was sufficient to exceed the limit of the brass strength, it seemed the full thrust was transmitted to the bolt, regardless of whether or not the case was lubed.
It should be possible to do a better job of measuring bolt thrust by some electronic means. Face it, my methods are primitive, arising, as they do, from a primitive mind, and require considerable interpretation which opens the door to all sorts of errors.
I look forward to MD's results and conclusions. GD

Greydog; Varmet Al did a finite element analysis which shows
that once the case head comes back the "bollt thrust" is
the same oiled or dry. I played around with some pressure
indicating tape and the results supported his conclusion.
I have a 760 Remington 35 remington with a tad of headspace.
By loading heavy the primers will not protrude.
Some of the old timers used an oiled case to "proof"
the actions. I believe what they were doing was making
certain that the breech locking worked.

Varmint Al has done some tests regarding this.

http://www.varmintal.com/abolt.htm

He removed the locking lug from the Winchester that he performed this test on.

Just cut them both off and see how far the bolt flies


W

Yes that was it. Thanks plainsman456.

My understanding is that some cartridges grip the chamber walls better than others allowing pressures to drop more before they shrink back down as pressure drops. Case head size matters too. The larger the case head, the greater the bolt thrust.

What about a rebated rim where the rim is smaller then the web area of the case acting on it?

It's the I.D. of the case head that pressure acts upon. The o.d. is not a part of the bolt thrust equation.

Good shootin'. -Al

Hawkins,

I tried the same thing with pressure sensitive tape. Did you have any problems with complete wash-out of tape indicating an exceedance of its indicating range? Just curious as I was hoping someone else had given it a try.

I had several problems with the tape but never a "wipeout".
What it did was seem to confirm that a dry and an oiled case
had similar effects above a certain pressure. My figure of
40 KPSI was back calculated from the velocity using the NRA
formula. How accurate ??. Anyway If Ackley had loaded his
30/30 improved a litle hotter he would have discovered why
Browning put the locking in the 94 Winchester.

Pressure in a 'vessel' acts equally in all directions, So, if the cartridge is at 60,000 psi, the forces is acting on the bullet the cartridge head and walls ....equally. The only 'thrust' you may get is the pressure forcing the cartridge brass into any voids ( like the clearance between the case head and bolt face).
So, if you have a crush fit of your brass and an action/chamber able to contain the pressure there will be nothing happening other than this force acting on the only movable part which is the bullet. F=PxA

The steel the action is made from will deform under pressure so there is also movement in the action. This movement can be significant in a rear locking action.

From reading the linked page of Varmint Al's, he seems to indicate that having the case head contact the bolt face early and fully is a good thing. Thus, a low CoF chamber is a good thing. Did I mis-read or mis-understand?

Yes and no. Under normal conditions, the ID is the more important. When the case head separates, the OD counts. A gun needs to be designed for the worst case.

P*A based on the ID is pessimistic, of course. The pressure blows out the case walls. These cling to the chamber. As the base stretches away, the brass on the walls holds it forward, offsetting the pressure some.

The link posted to Varmint Al's site wasn't the most relevant to this discussion. This page covers case cling.

Also, it wasn't just the "old timers" who oiled cases. UK proof loads using the base crusher system used oiled cases.

There is a bunch of BS on VarmintAl's site, such as:

"The fluted barrel with a 1.222" muzzle diameter has approximately the same weight as the original non fluted barrel but behaves entirely differently in dynamics from the chamber pressure and recoil."

I don't care how much education he has, or how many formulae he throws around. If he found chamber pressure and recoil to be different in a fluted barrel, there was another reason than the flutes.

I think the example cited was just sloppy English on his part--not unusual among engineers. I think he's trying to say "the dynamics which result from chamber pressure and recoil."

As for his education and formulae, his experience in applying them is what counts. The stress analysis software he uses is not for amateurs.

Key words ....able to contain the pressure.

For purposes of calculating bolt thrust, the i.d. of the case head is the only 'case head' consideration in the equation. When spining off to different 'what if's', certainly other things enter in.

But that's not how to calculate bolt thrust.....

Good shootin'. -Al

I disagree.

Bolt thrust is directly related to case head diameter as it acts upon the bolt face. If a 223 Rem creates 60K, the bolt thrust is less than a 300 WSM at the same pressure. When case head diameter, thus surface area, increases the bolt thrust increases exponentially.

2525,

You're probably right about wording being the problem, not the engineer or program. Thanks for the insight.

But wouldn't the internal case diameter defining the normal pressure plane also be smaller in the 223 as compared to the 300 Wizzum thus the lower load on the bolt face albeit same case pressure?

In my simple and empirical way of thinking, the case head statics could be crudely thought of as a pressure intensifier...or de-intensifier.

Yes and no. The main gas component is from the ID, but the case cling is a (not easily calculated) function of both the ID and the OD.

When the primer lights the fire the very first reaction is the case head slamming backward into the bolt face. The pressure curve, in my opinion, would have the lesser effect of the two.

This was my thought.
The ID has an area of X where it transfers pressure to an area of X-y. The result should be a greater area of pressure pushing on a smaller area of bolt face then if the rim was of the same ratio to the ID of a std rimless case of equal ID size. Sort of the same as a knife blade concentrating mild thumb pressure to a very small target area and severing it?
A large(r) inside area under pressure, on a smaller area of outside contact.

I always lube my cases and get indefinite case life that way. My theory is that the lubed case still grips the chamber walls but settles back more gradually and spreads the case stretch along its length so no part of it exceeds the elastic limit of the brass. By setting the case head firmly against the bolt face early in the pressure curve, the shock impact is reduced. Meaning that a case that grips the chamber walls too firmly will suddenly let go and slam into the bolt face with high impact energy. Think of a hammer blow. Too much lube will float the case and inject atomised oil into the chamber and take up volume so only a light smear of lube is indicated. I also fire-form my cases with an O-ring in front of the rim. Pre-loading the bolt is a great way to reduce shock loading.

Excess head space increases that hammer blow significantly.

On the transmission of pressure into thrust on the bolt face, it is the internal case base area, not the case head area that controls the thrust.

Several years ago I was doing quite a lot of shooting and got mixed up with the guys that were moly coating their bullets so I decided to follow suit and do the same. But then I committed the terrible sin of coating a batch of new brass, not knowing until after I had shot a bunch of them, that the case would not be able to get any traction and would hit the bolt face with such force that my gun would be blown to at least a jillion pieces. Well, in my rifle the case head was already against the bolt face and coupled with the fact that I am not very astute (Ringman's word), nothing of the kind ever developed. I was disappointed, to say the least as I had never had that experience.

As far as I know, nothing changed. I did get very good case life but I don't load em up to much over max anyhow.

Mule Deer; You said there was a lot of BS on Varmet Al's pages.
What else did you "detect" ?.

I'd like to know as well

As 2525 pointed out to me earlier in this thread, engineer English doesn't always communicate exactly what the engineer is trying to say. I found some examples of that, and incorrectly assumed the sentences meant something other than Al intended to say. Once I looked at them that way, as an editor trying to turn technical writing in English (which I've done in the past), it was apparent the BS came from the writing, not Al's technical information.

You should try working with eggineers on a daily basis...

You want me to do WHAT????

I appreciate that. I've always found his site informative and the technical information on it is so far over my head that it could actually BE BS and I wouldn't know better. I like the fact that he is a hunter/shooter with a vast engineering and technical background who ties them both together on his site. It's really a quite interesting site.

Yep!

I agree..it's quite an informative and insightful website.

His FEA analysis was quite impressive IMO. I've done enough FEA on my own behalf and worked with enough FEA results to appreciate one thing above all..."garbage in = garbage out". Whether it's meshing, assumptions, constitutive parameters, material properties, etc...it's easy to see if an engineer is faking his way through the model or knows what exactly he's doing...I'd conclude Al is on top of his game.

And yes, we engineers aren't exactly stellar communicators and we do ask the impossible.

The only problem lies in the common engineer's difficulty in equating engineering theory into real world effect. In other words, an engineer's assertion may be entirely valid but some aspect of the real world application may escape him or her entirely.
I well recall the time an engineer wanted me to install a buttplate of some miracle polymer on his shotgun because this material had the capability of redirecting applied force. This was undoubtedly true but the apparent hardness was about equal to a piece of hickory. My attempt to point out that the material had to be sandwiched between two pieces which were harder and less resilient than the super-stuff fell on deaf ears so I installed the "pad". The next day, the noticeably injured engineer came in for a Pachmayr pad. GD

In my experience, you are absolutely spot on. Some of the best engineers I've worked with have a background of growing up knee deep in schit and hands on experience...they are not short of common sense. On the other hand, some are only book smart.

There have several mentions of elastic limits on this thread. I thought I understood, but I am not sure.

I know elastic limit means that it streches so much that it holds that shape and does not go back to the original shape.

On a case with 100,000 PSI, the 100,000 PSI would probably be over the elastic limit. However the action was very strong and had very good gas sealing, in the event of a burst case.

That brass is subjected to 100,000 PSI, but since it is tightly sealed in a very strong container (the action) the brass neither expands nor burst.

This is disregarding that the action (container) might strech slightly, allowing the brass case to expand a like amount.

Would applying that much pressure to a piece of brass, although the elastic limit was reached totally remove the elasticity of the brass?

Would the compression of the molecules or what ever brass is made from cause a loss in elasticity, although the brass case did not expand and did not move and stayed togather?

I guess what I am asking is does it take strecthing to exceed the elastictity limit, or can pressure only render the brass unusable?

Elastic deformation stretches and returns to its original shape. Plastic deformation stretches and holds its new shape. Plastic is where you reach the limit that doesn't harm the material.

So JB, et al... How does Bolt Thrust affect shooters?

123457,

AJ300MAG covered the elastic/plastic description of alloys that demonstrate some level of ductility very nicely..brittle alloys (< 1% elongation for example) often show almost no plasticity. If you reach strains past the elastic limit and venture into the plastic regime of an alloy, permanent deformation occurs with only a slight amount of elastic recovery during unloading (amount of recovery is based on the modulus of the material). The elastic-plastic response of alloys comes in all forms...significant strain hardening, moderate strain hardening, elastic-perfectly plastic, etc. Most alloys that come to mind have a linear elastic regime governed by Hooke's Law. If you want to model the plastic regime along with the elastic regime..then your looking into a Ramberg-Osgood relation or something similar. Keep in mind most of what I just said is in a tension state....compression is a little different.

Good question and I think I understand what you're asking. Hopefully others will chime in as well. Here's my take on your question. I'm also inclined to think those with a lot of fire-forming experience will have some good real-life/empirical insight to your question as well.

100 ksi is certainly over the yield and ultimate strengths of cartridge brass. As you noted, however, it is constrained by the stiffer and stronger chamber. Comparing cartridge brass and typical ordinance grade steels, the steel is both about 2+X stronger and 2X stiffer (30 Mpsi vs 14 Mpsi) in the elastic response. From the brasses perspective, the scenario you outlined is a high pressure fire-forming event. The brass is going to flow and fill gaps between it and the constraining geometry of the chamber with plastic deformation certainly occurring. How much plasticity/straining will depend on much filling occurred. With plastic deformation comes working/strain hardening of the brass and consumed ductility. The often discussed annealing can/is then used to return ductility to the brass.

Under EXTREME uniform compressive pressures (think hydrostatic forming), unique things happen to materials that aren't up to the task. I'm not sure if 100 ksi is in that regime with regards to brass. Microvoids get filled and essentially your effectively compacting the material...probably so little it's insignificant. But, Poisson tells us that if something is "squeezed" to an extreme level, then a conservation of volume has to occur and you can actually fail the material by creating a complex multiaxial stress state. No doubt the behavior of brass in a chamber is a complex scenario with stress states all over the place.

To hopefully finally address your inquiry, the linear elasticity will always be there as long as the brass from a integrity level is still there (no tearing, cracks, etc). What you do encounter is often called consumed ductility (strain hardened the brass). If you could stress the brass up just before the point of failure and then unload the brass, any subsequent loading would show no signs of plastic deformation up to failure because it has been consumed on the first stress event. The oil industry has spent big $$$$ on understanding consumed ductility with regards to offshore pipe reeling...I've done more elastic-plastic tests than I care to admit; if I see another low-cycle stress-strain curve I just might puke.

Good question and hopefully someone else can add more.

Engineers here from China and India are often in this category--my wife for instance. These days, a surprising number of U.S. born engineers are, too.



It's only a concern when you are planning to rechamber a gun in which the action isn't the very strongest.