Let's Talk Bolt Thrust

[Sixgun]

Griff,
Should have bought a Colt. They shoot moa out of the box....and that's with a 16" barrel. No matter how hot it gets, it shoots. Forget that, on to business:

Somethings are very frustrating and it's always things you don't know nothing about, like computers. If I have an issue with a gun, I can work on something like a little spring for 4 hours and remain happy. But computers.......modern cars....women.....

I really made a cool video, about 2 minutes long and showed the Colt real close when it went off.......and I can't get it to upload. I even did a repeat using my I-pad and you tube does not recognize that at all. Now I know why I used to drink a lot, if I could not get something to work, I smashed it. Sure made me feel better......still thinking about that 750 Holly double pumper on a big block Chevy.

[KWK]

"Bolt thrust" is... directly proportional to the gas pressure generated and the area of the case head supported by the bolt or standing breech.

If you have any solid, empirical evidence to back up that assertion, I'd like to know about it. Shooting a marginal rifle in a mid-power cartridge, I'm always on the look out for information on breech thrust, but hard data isn't growing on trees. As I said before, I really need to try out that method Sisk has used.

If by your statement you mean pressure times the ID area of the case, then what evidence I've come across says otherwise. The British base crusher system clearly demonstrates case cling is a big factor. As mentioned, oiling the case changes the indicated breech thrust by about 25%. Further, using crushers to measure breech thrust allows one to estimate case pressure, yet these estimates were 10 to 20 % below what using a crusher to measure the pressure radially via a hole in the case showed. Again, case cling is to be suspected.

The best single work on this matter I've come across is Varmint Al's analysis of the effect of case cling in a .243 Win at mid and high pressures. It's a big page with much information to digest, but with a ruler and a calculator, one finds that over 1/5 of the breech thrust (as computed by case ID) is carried by the brass into the chamber walls. This is in line with the British numbers, as compared to radial crushers.

His analysis was done with a finite element system capable of estimating plastic strain. Numerical analyses are not perfect and can't be taken literally without backing by proper lab stress measurements; however, besides the British numbers, Al's analysis includes an estimate of case head expansion, and those numbers are in line with some measurements I've read about (I don't use the method myself).

In looking over Al's data, one sees a pitted chamber at 35,000 psi in principle can operate without the brass ever touching the breech yet stretching to the point head separation will occur with enough reloads. That doesn't mean there won't be any breech thrust. His estimates also indicate that once about 2000 psi is exceeded, the wee "piston" of the primer starts to exert significant thrust.

The thrust of the primer suggests that only the wimpiest loads on thinner brass in poor chambers might make zero thrust -- say a .22 Hornet firing a round ball with a puny charge. Otherwise, there will be enough thrust to start moving the breech.

Regarding the blowback pistols, Al's analysis gives no direct answers. The case he analyzed is long and has shoulders to push forward under pressure. Still, it shows the case base grinding rearward by the time 5 ksi is reached -- well below the operating pressures of such pistols. My guess is the typical, short blow back case moves back at lower pressures than that.

Sixgun's information on the Lightnings is also relevant, but I'm not sure what to make of it.

[Sixgun]

KWK,
I made a very unscientific test today with the Colt. Check out my post titled "Video of Bolt Thrust". Let me know your deduction.----6

[Old Savage]

Just how are we defining bolt thrust. Seems whenever there is push in one direction there will be push in another/opposite. And how did the premise get from the hydraulics of the inside of the gaseous cylinder to the size of the case head which should be irrelevant to total thrust delivered? And isn't the issue of bolt thrust just relevant to what will stop the thrust such as locking lugs? Clear me up here fellas.

[bgmkithaca]

From what I read here the consensus seems to be that bolt thrust does not seem to exist in lower pressure loads. That is not true-case in point if recoil operated shotguns operating at 10,000 to 11,000 psi would not cycle is there were no bolt thrust. If pressures get too low they won't cycle completely, but they will still open up partially. For every action there is a equal and opposite reaction unless the laws of physics has been re-written. Another angle to consider is the case wall gripping the chamber-yes, it does grip the chamber wall when enough pressure is developed. Plastic shotgun cases have a lower yield strength than brass and grip the chamber at lower pressure than brass but the shotgun still cycles.There is obviously bolt thrust present.

[w30wcf]

This has been an enlightening discussion.

KWK, thank you for the very enlighteningl information you provided.
Sixgun, your experience with the CLMR shows how low the bolt thrust can be in certain cartridges.

bgmkithaca,
Bolt thrust does exist, the question is how much? In the case of a shotgun using plastic hulls, the plastic would have a low coefficient of friction. I wonder if a brass shot shell would still function(?).

Then we have the semi automatic center fire rifles that need to use gas assist for positive ejection.....

w30wcf

[.45colt]

buckeyshooter Wrote"Well for me. If Nate Kiowa Jones agrees with Buck Elliot, I don't need popcorn cause the question is answered! Are you hungry for popcorn perhaps? ". Yep, and the show is starting to get Good Now. .

[bgmkithaca]

Yes, brass shotshells do cycle recoil operated shotguns and paper does too. As to auto loading rifles using gas to cycle it is more to unlock the action than anything else-once unlocked it will cycle but not just because of bolt thrust-there is a mechanical assembly in motion to help out. The gas pressure in the chamber is pretty well dissapated at this point. One thing about brass shotshells that can cause problems with ejection is balloon type heads-the ejector may tear through the rim as they tend to be pretty thin.

[bgmkithaca]

Some where in my archives is a formula for calculating bolt thrust. ballistic labs and engineers use it for development. If I come across it I will post it.As I recall
the math is pretty easy to use.

[w30wcf]

"As to auto loading rifles using gas to cycle it is more to unlock the action than anything else-once unlocked it will cycle but not just because of bolt thrust-there is a mechanical assembly in motion to help out. The gas pressure in the chamber is pretty well dissapated at this point."

Thank you. Yep, I knew that, but my excuse is that my brain wasn't quite awake yet when I posted.......

I don't currently have possession of the PO Ackley books but as I recall in addition to the test he did with the .30-30 and the excessive headspace with the result that the primer backing out kept the cartridge case in the chamber as he successively increased the headspace, there was a pic of a blown up M' 94 .30-30, The blow up was caused by too much fast burning powder with the result that the chamber area blew out in several chunks and splayed the threads in the action. PO stated that "The balance of the action was in perfect condition." No doubt multiple firings of a load at that amount of pressure would eventually trash the action.

No question that there is bolt thrust, the question being how much for the low pressure cartridges......
w30wcf

[bgmkithaca]

Here is the formula for bolt thrust:
Multiply the area of the case head in Sq. In. times the pressure of the round in PSI to get bolt thrust in Ft. Lbs.

Find the radius of the case head excluding the rim
A 45 Colt has a diameter of about .480"
The radius is .240" then squared becomes .0576 Sq. In.
Multiply this times Pi= 3.14
The area becomes .180864 Sq. In.
Multiply .i80864 times the pressure of the round to get the back thrust in Ft. Lbs.

.180864 Sq. In. x 8,000 PSI = 1447 Ft. Lbs. of back thrust.

Take it or leave it this is the scientific explanation-there are some academic factors as well but this will get you pretty close to the actual figure.

[AJMD429]

And how did the premise get from the hydraulics of the inside of the gaseous cylinder to the size of the case head which should be irrelevant to total thrust delivered?

You are right I think - the diameter of the rear of the cartridge/chamber IS going to determine the total pressure expressed rearwards, but the case head being large or rebated wouldn't make much difference unless the bolt were made of butter. The next stage in the handling of that pressure is going to be the bolt's locking-lugs, and the sheer mass/inertia of the bolt, and whatever friction there may be otherwise.

To the extent the cartridge case can be a 'gasket' and momentarily contain the pressure, IT may absorb lots of the pressure, too, depending on the degree of friction between cartridge and chamber.

It gets really interesting (and way beyond my abilities to calculate anything) when you take into account the dynamic situation, timing, metallurgy, and so on. It isn't just some static pressure-cooker popping out a relief-valve at some point, that's for sure...!

[AJMD429]

Well I better get some popcorn at the store today........................................................ .

[bgmkithaca]

And how did the premise get from the hydraulics of the inside of the gaseous cylinder to the size of the case head which should be irrelevant to total thrust delivered?

You are right I think - the diameter of the rear of the cartridge/chamber IS going to determine the total pressure expressed rearwards, but the case head being large or rebated wouldn't make much difference unless the bolt were made of butter. The next stage in the handling of that pressure is going to be the bolt's locking-lugs, and the sheer mass/inertia of the bolt, and whatever friction there may be otherwise.

To the extent the cartridge case can be a 'gasket' and momentarily contain the pressure, IT may absorb lots of the pressure, too, depending on the degree of friction between cartridge and chamber.

It gets really interesting (and way beyond my abilities to calculate anything) when you take into account the dynamic situation, timing, metallurgy, and so on. It isn't just some static pressure-cooker popping out a relief-valve at some point, that's for sure...!

The variables that you have mentioned are the "Academics part that I referred to plus a few more hair splitting factors that won't get mentioned here- the gist of it is the formula will get you close enough. The rest of the equation gets hairy and involved and won't serve this discussion much.

[Old Savage]

If a woman is wearing high heels or combat boots, the total downward push is the same though the lbs/sq.in. are different. Not buying the case head area idea. Seems if that were true, rebating the rims on rimmed cartridges would be like magic.

[earlmck]

If a woman is wearing high heels or combat boots, the total downward push is the same though the lbs/sq.in. are different. Not buying the case head area idea. Seems if that were true, rebating the rims on rimmed cartridges would be like magic.

Ah, but in this case we have lbs/sq.in being the same, so total downward push must be different. If the woman is in high heels then we've got the 800 lb. gorilla wearing the combat boots. The formula shown by bgmkithaca shows how it works: the pressure is the same in pounds per square inch. So the more square inches you have, the more pounds of bolt thrust. A 45/70 at 20K psi will have a lot more bolt thrust than the 22 Hornet at the same 20K psi.

But now for the "grab your bag of popcorn" part. bgmkithaca's formula assumes that the brass case becomes fluid and transmits all the rearward push. My understanding of our discussion is that there is a strong probability that under lower pressures that brass case isn't nearly as fluid as it is at those 60K psi levels and some lesser amount of thrust is therefore imparted.

Here I'll admit that I'd never given this any thought: I was totally in the camp that assumed the "lbs/sq.in X sq. in. = bolt thrust. Now I'm ready to be shown that this may not be all that correct at the lower pressures where brass acts more like a metal than a liquid.

Could I have another fistfull of that popcorn?

[Old Savage]

It is not psi on the case head. It is psi INSIDE the case at its base.

[Sixgun]

OS, While you educated guys are arguing about mathematical formulas I was running a brass rod down the bore of that Lightning....read about it on the video post.------6

[KWK]

... there are some academic factors as well but this will get you pretty close to the actual figure.

Without those academic figures, you'll be off by more than a factor of two!

That formula only works in an idealized problem where there is perfectly zero friction between the case and the chamber. No oil or grease can approach that. The only real world example is a case head separation. Chamber gases rush around the base, and an area of the chamber ID is presented to the breech at near chamber pressure -- more or less, depending on how well the breech is vented and how well the case rupture is letting gases pass by the case head.

A better estimation is to use the area of the ID of case just ahead of the case head. This is the area on which the gas pressure is shoving rearward. Case cling, though, results in this crude, "piston" calculation still being roughly 25% high. Well, that's what I'll use until I find proper test data.

In the matter of the rebated rim, the breech force is still the same, of course. The smaller rim might affect the plastic deformation of the case head onto the chamber walls, but I expect the difference to be quite small. The gas forces on the case head will be the same, and this will translate into the same breech thrust with either the rebated or the standard rim. However, the smaller rim will have higher internal stresses, reflecting the greater contact pressure between the smaller case base and the breech face.

[earlmck]

It is not psi on the case head. It is psi INSIDE the case at its base.

Yes, that's correct. And the inside of the case at the base of a 45/70 is a lot bigger than the inside base of the hornet case. Less bolt thrust from a hornet than for a 45/70, given equal pressure loads.

Or were you already saying that, OS, and I'm just misunderstanding? Lot of that sort of thing goes on in my circle.

If your point was that the bolt thrust is the same whatever the shape of the base: big rim, little rim, rimless, or rebated rim -- I agree.

[Old Savage]

We are on the same page Earl - Since Six likes a lot of technical info I checked this out with Dr. Mike Bramson, research physicist at the Air Warfare division at the China Lake Naval Weapons Center and he agreed with what you are saying - area of the inside cylinder base creates the force, he called it total impulse, area of the case head irrelevant assuming constant PSI.

[Sixgun]

We are on the same page Earl - Since Six likes a lot of technical info I checked this out with Dr. Mike Bramson, research physicist at the Air Warfare division at the China Lake Naval Weapons Center and he agreed with what you are saying - area of the inside cylinder base creates the force, he called it total impulse, area of the case head irrelevant assuming constant PSI.

I tell 'ya OS. People like Dr. Mike Bramson really let me know how stupid I am.----Little Tiny 6

[Old Savage]

Six - You are too humble - believe me, he would really like to talk to you about guns and maybe visit "The Farm" - shoot some RB into the pond and all. You would like him, he is quite irreverent about some of the academic academics in his field, always in good humor about it. To him the rubber must always meet the road. He drives an old blue 71, Z71 Chevy pickup, at least that is what I think it is. I had a 68 6 cyl.

[AJMD429]

If a woman is wearing high heels or combat boots, the total downward push is the same though the lbs/sq.in. are different.