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

[LRRPF52]

Interestingly enough I too had doubts about "bolt Thrust" at one time, so I did the calculation running the cross sectional area of a case-body wall, and the tensile strength of brass. I had to admit, there is a meaningful degree of force required to stretch brass enough that it likely does decrease force on the bolt lugs' which, while no means the bulk of the force, it was a fairly nontrivial amount. I can't recall exactly, but vaguely I think it was in the 5-10% of force type of range - but don't quote me on that.

You’re talking about case obturation resisting bolt thrust due to it purchasing the chamber walls, effectively acting like a friction brake on the brass as it tries to flow against the bolt face.

Brass vs steel vs nickel-plated cases have different coefficient of friction in that formula, and case taper plays a role as well.

The more taper, the more force is exerted on the bolt face and lugs. 7.62x39 and .22-250 are some of the worse offenders in that aspect.

The lower chamber pressure 7.62x39 can have the same or more bolt thrust than 6.5 Grendel as result, though bore volume is much larger on 7.62x39 so it can drop pressure faster (.310” vs .264”).

The 7.62x39 conical shape focuses the force more than a straighter case like 6.5 Grendel. 7.62x39 is a much harder nut to crack in the AR-15 than people might imagine.

It causes more problems in the AK as well, compared with 5.45x39.