Firing pin, springs, FTF.

[South Prairie jim]

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.

I’m thinking that the extractor is holding the case to the bolt face with opposing pressure from the ejector, I don’t visualize the case moving forward much at all. Although I’m certainly no expert. Once the primer is struck the flash goes toward the powder I’ll suggest force has to be the opposite direction again holding the case against the bolt face. Certainly the weakest part of the case is the mid sidewall where we typically see case separation from excess sizing over time ( personal experience here), therefore can we theorize that the case expands to the chamber wall during ignition prior to extending in length towards the shoulder ? We know from experience that it may take more than one firing to completely fire form a case in length before extraction becomes difficult and set back is required. Perhaps herein lies a clue to the mysterious shrinking case.

[charlie b]

Yep, perfectly valid theory.

I've fired a bunch of cases that never had the shoulder set back (neck size only) and they work perfectly well after more than 20 reloads. These were with moderate to low charges. As you said, full powder charges tend to start 'sticking' after 4 or 5 reloads when a FL size is 'needed'.

FWIW, the times I had shoulder set back from a FP hit were with my cast loads. Those are bullets seated to a jam fit in the throat so there is no gap at the bolt head.

[South Prairie jim]

Yep, perfectly valid theory.

I've fired a bunch of cases that never had the shoulder set back (neck size only) and they work perfectly well after more than 20 reloads. These were with moderate to low charges. As you said, full powder charges tend to start 'sticking' after 4 or 5 reloads when a FL size is 'needed'.

FWIW, the times I had shoulder set back from a FP hit were with my cast loads. Those are bullets seated to a jam fit in the throat so there is no gap at the bolt head.

Yep, perfectly valid theory...

go figure..lol