Home Forums Hunting & Shooting Ask The Gunwriters Bolt thrust question

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.