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

[LRRPF52]

Been listening and learning and mulling this over and a technical question arises regarding brass materials science: What is the minimum pressure in PSI required to deform cartridge brass, to obturate it to the chamber walls? Can't find this answer by Googling, or my Google-Fu is weak.

This is relevant to what pressure might possibly be required to be residual in a case to cause the case web area to balloon into a "belted magnum" as it is in the process of being extracted from the chamber during an early unlocking event.

Apologies if this was asked and answered earlier.

Bill answered that:

Bill Alexander: Brass will move under a residual pressure of only 1600 psi. For a fully plastic movement it will obviously depend upon the composition of the brass but I would use a pressure offset between 5400 psi and 7000 psi while running pressure/velocity measurements. This was derived by static hydraulic measurement.

With that said you can now look at your rifle set up. The Grendel, (I have no experience of the 6ARC) we could time a gun to drop to the correct residual bore pressure with rifle length gas systems up to 24" barrels. This did however demand that the gas balance was rather finely set. Carrier had to be full weight and a minimum of an H2 buffer (with the tungsten nickel iron weights not the tungsten carbide weights). Hydraulic buffers created general mayhem and only the 308 type was suitable with shorter barrels or longer gas systems, if not both. I would expect from the expansion ratio that the 6mm would be more sensitive.

Statically, the breach has to be machined correctly. The case must remain supported to a point behind the start of the case web, seen as where the case walls begin to thicken into the case head proper. This was achieved by reducing the 120 feed cone diameter on the barrel which was in turn made possible as the round would lift higher on the feed ramps. The projectile diameter placed the centerline higher when it sat on the feed ramps. Again not sure how this would look with a 6mm. Note that the support must not coincide with the case wall to web transition or a shear line is created.

If your gun has the correct dimensional set up then my immediate suspicion would be residual bore pressure from the buffer allowing the breach to open too early. Hope this helps.