Originally Posted by
charlie b
The cast bullet folks have theories, but, they are just that. I have never seen conclusive proof of any of it. I have had case shoulder setback from just the FP impact.
One is that the firing pin shoves case forward and then the expanding gases force it back as the bullet is pushed from the case. Not enough pressure to 'form' the case to the chamber so the neck remains set back.
Another is the primer ignition pressure inside the primer cup is enough to force the case forward. The case pressure eventually pushes the case back over the primer, but, not enough pressure to reform the shoulder. These folks will drill out the flash hole to reduce the area over which the primer pressure acts. I don't know if that is the 'fix' or if the additional flame front acting on the reduced powder charges creates a higher pressure earlier in the cycle, which could prevent the setback.
Long ago there were theories about how a case reacts in the chamber. Case is shoved forward by force of striking primer, pressure builds, the thinner portions of the case 'grab' first, then the web and base are forced against the bolt head. (This is one that is used to support the mode of case failure when the case head separates) Aggravated if there is excess headspace or a case is resized a bit 'short'. But, some of those who size to an 'interference' fit still might see case head separation (I've never had it happen to me, knock on wood :) ) so is this a realistic scenario?
None of this is really relevant unless you are using fired cases and primers to deduce a problem with shooting. And that is complicated by how many times cases were fired/annealed, actual pressure curves, case volumes, case wall dimensions, sizing, etc, etc.