In my imagination, the case moves back to bolt face first, and the stretch / reform movement of the shoulder and neck take place afterwards.
This because the neck cannot expand until the bullet leaves and allows the internal surface of the neck to be exposed to the pressure.
But as I said above, the bullet ogive would have hit the lands and the pressure would have pushed the case back to the bolt face....being the line of least resistance...before the bullet has moved out of the case neck.
It is only as the bullet leaves that it progressively exposes the internal surface of the neck to the pressure allowing it to radially expand and grip the chamber wall.
Alan
I think that is all in your imagination.
And that Dalua has correctly described the mechanism that I think is at play. As I already postulated.
Don't think about the case neck, that is not what grips the chamber, it is the body of the case. Which at these pressures is not much stronger than a rubber balloon.
In many rifles the case will be loaded in to the chamber all the way, until it headspaces on the shoulder. If it has e.g. a plunger ejector that will have pushed it forwards when it was loaded. If it doesn't, the firing pin will probably have pushed it forward until it completes it's strike.
Either way, let's just assume that the round starts off all the way into the chamber, and that the headspace is the headspace, i.e. not a few microns, but several thousands of an inch. Typical SAAMI chambers and cases, in combination, can have headspaces of over 10/1000" and still be in spec. NB: this is not the headspace that you can measure with go/nogo gauges in the chamber. It is potentially more.
E.g., for the .308 Win:
Minimum case length to shoulder datum (1.634 - 0.007) = 1.627" minimum.
Maximum chamber length to datum = 1.640"
Thus headspace for a minimum cartridge in a maximum chamber is 13/1000" That's 0.33 mm i.e 330 microns. Assuming that everything is in spec. And who is to say that a reloading die actually stops at precisely cartridge minimum dimension if cranked all the way down ?
Go/nogo on the chamber is only 10/1000" tolerance. 30% less.
Gun goes bang, cartridge stays forward in the chamber, pressure rises inside case until high enough to overcome neck tension and the bullet start moving. By which point the body of the case will already have expanded until it grips the chamber walls tightly. The force (psi x area) to which it is applied, inside that thin-walled elastic cartridge body is far higher than that acting on the base of the bullet. And as the pressure rises further as the bullet is in the first couple of inches of the barrel, it will grip the chamber walls even more tightly.
The neck is the last part to obturate, it only does so, perhaps imperfectly, once the bullet has left it.
I am confident that this is correct. If everything is working correctly the case body will show no signs of sooting/blowby, yet it is common for the neck to be more or less sooted. This only works because the case body, and shoulder, has already obturated before the bullet has left the neck. Otherwise you would get soot all down the case body and shoulder (can happen if the loads are too light).
If the headspace is large, the primer will also be pushed out a little until it hits the bolt face, maybe flattening it. As the pressure in the case diminishes it will relax its grip on the chamber walls and only then be pushed back onto the bolt face, pushing the primer back in, but meanwhile the case may have already stretched at the head/web portion.
Consider, in the limit, with overpressure rounds, that it is even possible for the primers to blow out completely, leaving no trace. That is simply not possible if the case head was being pressed hard against the bolt face, nowhere for the bits to go. QED in that scenario there must have been a gap between case head and bolt face. Maybe simple inertia, primer blown back and out before the inertia of the far heavier case head, and even the rest of the case, if it hasn't already stretched at the web, has been overcome.
This also takes no consideration of any elasticity in the gun itself. Maybe not much of a consideration with those that lock the bolt at the front, but it certainly is with those using rear locking lugs such as Enfields, or lever actions that use a rising block instead.
Sorry, I don't have any slo-mo video to show this.
And, BTW, some military gun proof tests require at least one of the proof rounds to be oiled, to minimise grip on the chamber (anecdotally this can account for 30% of the bolt thrust), so as to stress the action fully.
Well, that's what goes on, in my fertile imagination.
Finally, Alan, how do you explain the well known phenomenon of case thinning at the head/web, potentially leading to separation ? In your your imagination that should never happen, yet it does.
OK, one last conundrum, consider the .303 British, of which I have some experience, since I have one. This is notorious for case head separation if reloaded without understanding the issues.
This headspaces on the rim. SAAMI specs. below, not that many battle rifles were built to SAAMI specs, but humour me.
So, the headspacing at the rim can be between zero and 17/1000" Sloppy eh. ?
And the fit of the case at the shoulder, ignoring headspace variation, anywhere between 0.019" and 0.049". Loose doesn't begin to describe this. Plenty of room for e.g. WW1 mud in there.
And weirder, the chamber shoulder angle is much sharper than the brass shoulder angle (these drawings are to scale). Who'd have thought ? Well the people who designed it did, and made probably the ultimate slick-loading battle round at the time, never intended to be reloaded.
With of course the rear locking lugs and bendy receiver in the equation.
So, what can happen ? Well if you just put the cases into a FL die, cranked down to the shellholder, then don't be surprised to get shoulder splits and head separations PDQ. In two or three reloads. Yet baby your brass, reload so it headspaces off the fireformed shoulder, use lighter loads, and it can last pretty much forever.
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