Home Forums Hunting & Shooting Ask The Gunwriters Bolt thrust question

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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