Home Forums Hunting & Shooting Ask The Gunwriters Bolt thrust question

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.