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

deersmeller, I recently ran a few basic numbers for a .45-70 case a friend was kind enough to section for me. Even down near the base, where the walls are thicker (in particular, at the bright ring on this case), the brass should begin to deform outward (towards the chamber walls) at under 7000 psi. At well under 20,000 psi, the case base would begin to stretch (a permanent stretch, ie. deform, therein making the bright ring) towards the breech. At typical operating pressures, the net case area (the bullet) will not determine the breech thrust.

The only persons who need to worry about calculating the work done on the bolt are the firearms designers, especially of blowback, roller lock, and gas operated bolts, where the inertia, spring and damper mechanism has to be matched to the cartridges which may be fired in the weapon. The issue there is not any instantaneous peak force so much as the increasing and decreasing forces over time so long as they drive the bolt rearward against the springs.

In the late 1970s, I developed a computer model of the M-14 in Fortran, because I had an M-14, and everything about it was known and worked properly. By changing springs and piston gas flows, I could test the calculation against the known real world. Then it could be applied to other firearms.

Thank you for this quantified example and for helping to resolve the misunderstanding there was between Tailgunner / Lee24 and me.

I think my physics is pretty sound and the rest is speculative as I have never measured it. This bolt thrust is more than pressure as pressure is equally distributed in all directions per unit area. The Newton third law and force come in to play.

Force is delta P or the rate of change in momentum (at rest or in motion), further, force changes or tends to change a state of rest or of motion, and will cause a body (mass) to ditort or move. Now the equal and opposite part, the force required to get a 100 grain 243 bullet moving is less than required to get a 250 grain 35 whelen moving, and the opposite force to the respective bolts will be proportionally different as well. All this is mitigated by the grip of the case to the chamber wall and the straighter the case the better the grip. I don't know how we would measure the difference in the grip but it seems that a crusher type system on the rear of the case head would reveal some variation as we change the angle of the taper of the case. I am sure the Ackley or similar styled cases have less bolt thrust than a more tapered case.

What do you think of this synopsis?

I lost part of this and wanted to add....with the comparison of the 243 and the 35 Whelen the whelen has greater recoil due to this equal and opposite force.

Two calibers with the same bolt head diameter and operating at the same pressure with the same case taper/shape will have bolt thrust proportionally to the force required to accelerate their respctive projectiles out the muzzle.

Murphy

I think you are missing some of the point here. No one argues that pressure is exerted equally in all directions. Just that the radial area of the circumference of the case is greater than the case head and more force is a applied to the sides than to the case head. The outward pressure expands the case against the side of the chamber and the pressure against the case head isn't enought to overcome the wall pressure and move the case rearward.

If you shoot a gas autoloader with slow burning powder you will see how strongly the case is held in the chamber. The slower powder is too high at the port and is too strong to release the case, so when the bullet passes the port the action actuates and the extractor will pull a piece of the case rim off and leave the case in the chamber, stuck by gripping the walls of the chamber. After we stop to clear the stoppage the case will come out easily. You can do this with a Remington 742/7400 with Rl-22 or H4831 powder but not with 4895 or 4064 powder, they will cycle the action normally. (This may damage the actuating mechanism) Works pretty much the same with the Garrand.

The point is that a case does grip the side wall of the chamber very tight, and straight case grip tighter. With a revolver, which has a lot of headspace to allow the cylinder to turn freely, the straight case grip the walls tight enough to keep the case from moving until the pressure drops(bullet leaves the barrel) then the spring action of the brass allows the case to contract back to original size except for a little permanent bulge. Bolt thrust is mitigated (reduced by this action, I don't know how much nor do I know how to measure it. Anybody else have an idea?

Murphy

I respectfully disagree. For equal heads, shapes, and pressures, the cartridges will have equal bolt thrust regardless of the weight or caliber of the bullet.

What's to tell the back half of the case the size and weight of the bullet? All the back half sees is the pressure, and you just stated that the pressure is the same.

KenO

Ken,

I will tip my hat to your well rounded experience and expertise. Can you explain how the opposite forces of recoil have no effect on bolt thrust. Also am I all wet about the case shape and grip of the chamber wall having an effect on bolt thrust.

Thank you.

I don't claim to be a physics prof, but here is my quick and dirty interpretation.

As far as the recoil goes, the primary consideration is momentum. Neglecting weight of the powder gas, the momentum of the rifle will be approximately equal to the momentum of the bullet leaving the muzzle. Considering the relative weights of bullet and gun you can get the rearward velocity of the gun and from that you can get the recoil energy of the gun. The influence of recoil on bolt thrust is negligable.

The business of "summing forces" or "equal and opposite" are generally applied to rigid bodies. In a cartridge case it is almost safe to assume that the cartridge walls are bubble gum instead of rigid brass. Pressure is transmitted thru that layer of bubble gum to chamber walls, bolt face, and directly to the base of the bullet. Neglecting motion, the pressure is the same at all points within the combustion volume. The same pressure applied to a larger area gives more force. That is why head diameter is important to bolt thrust, but makes no difference regarding pressure. The same pressure and the same head diameter will give approximately the same bolt thrust.

KenO

Ken,

Thanks for you response. I am with you on the recoil, what it is and how it is and the momnentum values.

Does negligable mean none?

Ditto; The pressure is the same at all points with in the chamber walls. Also, I understand pressure per unit area, larger head more force.

Does approximately mean same?

Not trying to be sarcastic here, it's just makes sense to me that the recoil pulse and bolt thrust (from calculated psi) would be additive and that (if) the cartridge case of bubble gun does not move against the bolt head because it is held by the internal pressure against the walls, that it would reduce bolt thrust by an amount, albeit small.

I cannot quantify these values just asking if they make any sense. Also to calculate bolt thrust with pressure and head area, it is valid when used as a comparison of one case to another such as 222 Rem vs 378 Wby.

Also do you think headspace, excess or not, plays into this equation at all?
Thanks,

Murphy

I'll try to find a thread that pertains to this topic. I'm not even sure what site it was on and it's been a few years.

A gunsmith had set up a way to fire cartridges, both standard and AI in almost just the barrel. All the standard calibers shot out the back at up to 2000fps if I remember right.

The AI cartridges never moved out of the barrel. I saved information on this, but I've moved since and will have to look for it.

Murphy
In my first post, I discribed using a lead ring between the bolt face and the cartridge base. IOW a crusher (knowing the origional area & thickness, the amount of deformation and the compressive strength of the material you can calculate the force applied). This isn't a "weird" idea, IIRC it is somthing the British used to do inorder to understand/quantify the bolt thrust of various cartridges. We get the "Area x Pressure = thrust" formulia from their work in this area.

The case can reduce the amount slightly, but only by the yeild strength of the case wall/case web junction (note that a typical military case is thicker in this area). A "Improved" case does nothing to increase the area of this junction.

Sorry for the delay in resonding. I've spent the last couple of days driving and in an ammo plant doing troubleshooting.

Negligable doe not mean none, it just means small in comparison.

Approximate does not mean same or equal, just close enough for practical purposes.

Headspace will affect bolt thrust, but I can't quantify it. Ken Howell had a beautiful quote from Homer Powley to the effect, "The position of the moon has an effect on exterior ballistics, but it is hard to measure."

From my perspective, recoil forces might influence bolt thrust, but the effect is so small that I won't worry about it.

Yes, the English used bolt thrust to measure pressure, but remember that their cases were well lubricated just before the shot.

Others might trust case strength and friction on the chamber walls to keep an AI or similar case in the chamber, but I certainly would not.

Somewhere between the the British saying that bolt thrust is proportional to pressure and head area, to someone saying that some cases are totally held in the chamber by friction, lies the exact bolt thrust. You must design for worst case so that the bolt will contain the highest possible bolt thrust. The one person I know who survived a bolt in the eye doesn't speak highly of the event.

KenO

Reminds me of an equally beautiful quote from Bill Davis. I don't remember the exact context, but it had to do with some trifle that had a shooter all worked up. Bill said "If you pee in the ocean, you will raise the tide, but does it really make a difference in the depth of the sea?"

A number of years back, Dan Lilja wrote an article for Precision Shooting on Bolt Lug Strength and included the simplified formula for "Bolt Thrust" which was the same as what's posted on this thread. (internal cross sectional area of case*pressure)
What I found interesting was that it assumed zero friction between the casewall and the chamber which he acknowleged as being significant, but for the purpose of firearms design is ignored. In fact the apparent convention in firearm design is to design in a safety factor of 2 with full backthrust (zero case/chamber adhesion)...I like that Dr Oehler <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />.

I'm surprised no one has invoked PO Ackley's writings in his book "Handbook For Shooters and Reloaders/Vol 1". In his chapter entitled "Pressure" (pp.134-149) he discusses case design's effect on backthrust and describes his various experiments that support his assertion that case design can reduce backthrust.

It interesting reading for me and I would guess for most on this thread as well.

Here's some thumbnail descriptions of his experiments;
Savage 99 in 250/3000 loaded to the point where action would lock up. Rechambered to "Ackley" config (only body taper changed). No extraction problems until primer pockets swell. (a different weak link becomes significant).

Winchester 94 chambered in 30-30 Ackley. Barrel unscrewed two turns. Dry round fired, brass maintained proper cartridge headspace in spite of increased chamber headspace. Primer backed out exactly the amount of the increased headspace. Oiled round fired; Case separated.

Same Winchester 94 with Ackley Chamber. Fired with Locking lug completely removed. Bolt held closed only held forward with finger lever. Fired cases are normal except for excessive primer protrusion.

Here are some quotes off the chapter;
"...This method gives a fair ideo of the actual chamber pressure exerted upon the walls fo the chamber in all directions, but bears little relationship to the amount of this pressure actually transmitted to the bolt or breech block in the form of thrust. The English method actually measures actual thrust against the bolt...which in turn is converted to breech pressure."

"...By studying these pictures it can easily be seen that the brass cartridge case WILL withstand at least SOME pressure, that the minimum body taper takes the "load" off the bolt or locking system, that oily chambers increase the bolt thrust...."

Finally, I found that Harold Vaughn had done experiments with strain gages to document bolt thrust in his search to eliminate receiver error( "Rifle Accuracy Facts", pp55-58). Similar to Ackley, he found that Oiled cases produced significantly greater backthrust than Dry cases (7500 lbs. versus 800 lbs in his .270 chambered Remington bolt), primers backed out proportionately with excess headspace ala Ackley.
Vaughn says Zero headspace produced maximum bolt thrust verus excessive headspace. In his text he lumps it with the oiled versus dry case numbers and I couldn't tell if he isolated the effect (though I suspect he would have to have made the statement).

Oh Yeah...
For those that want to get neck deep into the effect of bolt thrust on the lugs, from Al Harrall;
http://www.varmintal.net/abolt.htm

...I just like the pictures. The words make my head hurt.

Neat stuff. We've come a long way with our abilities to model and render 3-D phenomena. The renderings would be a little more informative if we could have a look at the rear surfaces of the bolt lugs that bear the brunt of the load (no pun intended). 1Minute

Ken,
I've owned and fired an even dozen Ackley Improved calibers, and of them five different 30-06 AI. I have used Oehler Chronographs sense the model 10 (Actually made my first one and used your little wire screens) This last 30-06 AI, Shilen barreled Mauser 98, a keeper, has this to say.

Bullet: 180 grain Norma Vulcan.
Case: Winchester.
Powder Alliant RL-19.
Primer: Fed 210M.
Chronographs: Oehler M35P (both)
58.0 grains Chrony #1 2817 fps Chrony #2 2798 fps
59.0 grains #1-2853 fps #2 -2849 fps
59.5 grains #1-2883 fps #2- 2873 fps
60.0 grains #1-2909 fps #2-2898 fps (high press.)

Powder: H4350 Extreme
56.0 grains #1-2773 fps #2-2759 fps
57.0 grains #1-2832 fps #2-2828 fps
58.0 grains #1-2853 fps #2-2851 fps
58.5 grains #1-2893 fps #2-2880 fps

The shots were fired through two completely independent M35P's on the same six foot piece of 1/2" emt conduit. #1 was the front with front and rear screens 2 ft. apart and #2 was next at same screen spacing 2 ft from the first.

So...Sense there is no way an 30-06AI can give these velocities with this powder charge. The chronographs lied.

Or....sense there is absolutely no difference in the bolt thrust (with same pressure) between the AI and the std -06, and sense there is less case expansion than factory Winchester ammo (180 grain cheap stuff Chronoed at 2683 fps) and case fall out and primers don't even have a slight hat brim and case head shows absolutely no sign of high pressure (I've looked at 400,000 handloaded, fired cases), then this must be a myth. Maybe Brian is right, this is a myth!

This isn't the first such experiment or experience I've had in this world with guns, or shooting or loading or clocking through the triangles of the 35P. (and others)Everything has led me to some conslusions. I understand there are many variables in any experiment and I do understand how some rifles are just different from others. But these many experiments with different case shapes straight vs tapered have led me to believe that what we see on the cartridge head is masked to some degree from the case of an AI, or similar, and can't be relied on solely for pressure signs.
Is there nothing to what I've believed valid for so many experiments? Maybe I should have taken up golf. Why did I work so hard to be educated, I could have been a beach bum! Can you throw me a crumb, here?

Thanks very much for your help and the excellent tools.

Chris,

I like what you wrote..... I don't remember seeing that in print before, but it fits with what I've carried around in my mellon for all these years. Maybe Parker was the source for me as I've read his books, some time back, though. After all this about the bolt thrust and Ackley myth stuff, I went out and reshot and borrowed a second Chronograph for this experiment. I also shot these same loads in a standard -06 as far as I could before pressure got me, and recorded the velocity and the case head expansion. A whole week end doing what I had already done. Same results. I think the Aclkley (or similar straight case) reduces bolt thrust.

I'm not designing rifles here and I can do the math for the bolt thrust calculation, I'm just saying it ain't just that simple value.

Thanks for you comments.

Well, I certainly can't fault your taste in chronographs, and I envy the velocities you are getting with your rifle.

I'll concede that the 400,000 fired cases you've observed is probably 100 times as many as I've looked at. I've probably only looked at a few thousand after they were fired in the course of a pressure measurement with one technique or the other. I've never had the chance to personally observe the British system.

I've never been able to establish a reliable correlation between primer appearance and pressure.

From what I've been able to glean, most significant head expansion takes place only after factory max ave pressures have been exceeded. That's when I gave up. I learned long ago that just because I can't do something doesn't mean that lots of other people can't do it. Perhaps you can point me to a reference that documents the correlation between head expansion and measured chamber pressure.

The simplest relationship I've consistently seen is that if a shooter works up a load by the usual "increase charge slowly until you see some signs of pressure on the case, and then back off a little bit", then the pressures with typical brass will usually indicate 70K - 75K by the time we put a strain gage on his gun. This pressure is typically between max average pressure and proof pressures. Continual use of loads with pressure falling in this range must be a judgment call by the shooter. I'll pass.

Perhaps the title of this thread should be "Can "Bolt-
Thrust" be reliably predicted?" We can measure a lot of parameters that we can't predict, and I suspect that the bolt-thrust is one of them. We can measure bolt-thrust, but we can't predict it unless we oil the cases.

Cartridge shape does have an influence, but I seriously doubt that it over-rides head diameter. Just how did the British measure pressure of the old straight sided rifle cartridges by looking at bolt thrust? Lubrication is much more important with the straight cases that tapered ones, and I certainly don't know how to consistently predict the "slickness" of a lubricant or to assure that the chamber and case are absolutely clean.

KenO