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

[LRRPF52]

Would the brass not get weaker as the walls thin due to stretch and expansion that occurs concurrently during an early unlock, where peak yeild pressure is measurably reduced due to changes in material dimension?

I'm not a mechanical engineer, maybe you can clarify. If there is no failure, then what is considered peak pressure in the formally trained sense as described previously? Just some greater than x equation?

You could have the strongest brass in the world (Lapua), but if you retract it early while there is sufficient residual bore pressure, the moment enough of it clears the chamber walls, it will expand because nothing is there to support it.

A great way to create that condition is to use really light weight on the reciprocating mass (carrier, buffer, or both), shoot a long bullet with slow-medium burning ball powder, with a CLGS port location.

Cartridge brass will move around with as little as ~5200-5800psi if I recall correctly.

We had the answer to the OP’s problem long ago in this thread, and now I’m getting upset that I missed it.