I had a partial case head separation using a 1.8oz buffer

[DeNinny]

If the brass yields in-battery, it will happen more as a result of peak pressure, not residual pressure post-exit of the projectile.

To me you are claiming the effect is the cause here. "Peak" pressure is completely over as soon as either 1) the bullet leaves the mouth or 2) the blowout occurs.

The blowout occurred because pressure was at some level inside the casing and it simply reached the necessary force to rupture the brass. This is MY definition of "peak" pressure. The laws of chemistry and materials science dictate this.

The reason a metal deforms is because force (or pressure) is applied to it. And all materials, metals/alloys in particular, go through a phase of temporary deformation. And then if enough force is continually applied, the metal will reach a point of permanent deformation. At this point, it will never restore itself to original shape. And then if even more force is applied, the metal eventually ruptures. And then once that rupture occurs, the metal wall itself is no longer holding back all that pressure so it immediately dissipates. The pressure will not ever "peak" after this.

And in this failure, as soon as that rupture occurred, at whatever peak pressure it took, the pressure dropped. This is how the pressure will respond in any fully pressurized vessel that is pushed to the point that a weak point in it permanently deforms and ruptures.

So with all this, the pressure was not "residual" as you stated. The pressure was rising with the cartridge in battery, it simply reached the point of stretching the brass into the feed cone, at which point it started permanently bending to the shape of the feed cone. And then finally, the force (pressure) reached the point of tearing it. And since a bend is a weak point in the alloy, the tear started there. This is all from the pressure within the case that started building as soon as the primer fired. And it kept building until the brass ruptured.

And remember, immediately after the tear started, pressure started dropping rapidly. Plus again, the bullet leaving the mouth dropped it a lot too. So there definitely didn't need to be all this other extra pressure to do all that damage as you are assuming would happen.

The pictures are evidence that the brass ruptured in battery as per all my other points. Above is the chemistry and material science theory of how it happened and is all 100% consistent with what I see in the pictures. Every blowout will happen the same, and yet the peak pressure at which it happened doesn't have to be exactly the same.

To me you are assuming all blowouts have to have nearly the same damage. The science to me says otherwise. You have to analyze each scenario in depth to flush out those differences.

The farther the projectile moves from the chamber, the lower the pressure is.

Agreement!!!

If the brass yields in-battery, you will see substantial damage to the action.

Not necessarily per all my points above.

I would expect to see this with a case yielding in-battery:

...

When extractors appear to stay intact with an in-battery case failure, they look like this:

...

To me these are all just higher pressure failures. The OP's failure just happened at a lower pressure which still happened in battery. Clearly and lucky for him!

But what we have seen is not indicative of a case failure in-battery.

This is exactly what the pictures support and as I have described. IMHO.