It seems to me that if you have a case head seperation that the bolt would be subject to thrust based on the full diameter of the case X p.s.i. of the case. It also seems to me that if the tensile strength of the brass is exceeded that the case taper would not matter, the case stretches at the thinnest part of the body regardless of the case taper ahead of that point. I can see how a straight walled case that did not exceed the brass strength would minimize bolt thrust. I've had .30-30 fired cases (Savage 340) that had the primers sticking out quite a bit. I suspect that the h'space was quite large and the pressure was quite low. Just some thoughts that I've had after reviewing the discussion below that degraded. Please, no flames, just comments on my wrong thinking.

Hello Jay - there is a range of stresses where things stretch and then return to their original sizes. If you stay within this limit nothing exciting happens. The case stretches and the load required to stretch the case is load that never reaches the bolt face.

Increasing headspace and staying within the elastic limit of the brass would reduce the thrust on the boltface still further.

Continue increasing headspace and you enter the region where the brass deforms permanently. This is shown by that little frosted ring on the outer surface of the case and a groove on the inside of the case under the frosting. This is the area where the stresses exceeded the elastic limit of the brass and, as a result, it becomes permanently deformed.

At this point the rear of the case is against the boltface and due to the stretching of the case you are beginning to transfer more load onto the boltface.

In severe cases the brass comes out of the chamber in two pieces. At failure you have now lost all reduction of load to boltface due to stretching of the brass and the boltface has to handle the maximum thrust.

So you end up with a rather funny conclusion - as you reduce the headspace, the thrust on the boltface increases.

Let me leave you with something to think about in respect of case tapers. A Morse self-holding taper is generally given as about 0.625"/foot. The Ross 0.280 case has a taper of 0.602"/foot. Would you only neck size these cases?

cheers edi

edi---

I think you have a flaw in your theory.

As excess headspace is increase it adds *motion* to the case as it expand back against the bolt face....(remember, the primer drives the case forward, then the case is blown back against the bolt face at firing.)

When there is motion to the part there is *kenetic* energy in addition to static.

This fully explains why an action with excess headspace will continue to set back at ever increasing rates until there's a failure or the lug set-back becomes bad enough to lock the bolt in place at firing.

One of the more interestiong experiments is the addition of pressure sensors to the bolt face (the Brits had a replaceable bolt face with a ring-style copper crusher pellet under it). The change in stresses at the face can be easily measured from increasing headspace, oily brass, straight walled cases, etc..

Jack,
The kenetic energy scenario is valid only after the pressure reaches the point that the elastic limit of the brass is exceeded to the extent that the brass does indeed contact the face of the bolt. In a situation where the primer is left protruding it is apparent that the case probably did not contact the face of the bolt. I say "probably" because it is possible, of course, that the case did contact the bolt face but returned to it's original dimension and the primer is protruding due to the effect of residual pressure. I personally doubt this however.
In Jay's scenario where there is a complete head separation it would be true that the pressure would now be acting upon the entire diameter of the case rather than just the internal dia. In other words, the separated head has become the piston and the pressure acts upon the sectional area of the piston. It makes no difference what the shape of the piston may be. It may be domed, or dished, or stepped, and the transfer of force is the same.
Jay,
You have likely read my feelings on the matter of taper vs bolt thrust. I contend that once the case grips the chamber walls sufficiently to allow the case head to separate, no change in cartridge shape will make any difference whatso ever in bolt thrust. In other words, I think Ackley was wrong. (Flame on folks!) In the case of your Savage 340 the question is, "as the pressure increases does it overcome the ability of the tapered case to grip the chamber walls or does it over come the elastic limits of the brass and separate the head?" I believe the latter unless we introduce lubrication to the mix. (By the way, Dad cracked the locking lug off one of these by the simple expedient of rechambering it to 307 Win and shooting factory equivalent loads.)
I think the british system was not bad but was somewhat flawed since as the ring compressed the tensile strength of th brass would become a more important part of the equation up to the point that the case failed. I have explained my understanding of the dynamics of energy transfer as affected by lubrication or the lack therof previously and won't bore you with it again! Regards, Bill.

Thanks Bill, I was wondering how to word my reply without introducing the concept of the "fast" world of combustion dynamics.

The cartridge case is hard up against the boltface almost before the bullet has moved 3" and with the peak pressure still some time in the future.

But please don't say that Ackley had it all wrong. Provided you keep everything within the elastic range his comments remain valid.

cheers edi.