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

[LRRPF52]

And another post directly addressing Lapua brass and how it articulates with the chamber under initial peak pressure, as well as during extraction:

Bill Alexander: The Grendel chamber geometry and bolt was designed in concert with the Lapua case. The chamber "if" correctly dimensioned and with the correct feed cone geometry will allow chamber pressures well in excess of 68k psi without the case deforming. Obviously this is for Proof loadings only and should NOT be used as the basis of any load development. It will damage the rifle! The absolute maximum is 51,800 psi and MAOP pressures are held around 50k psi. Pushing loads to 52kpsi allows zero envelope for pressure excursions, which will and do happen even with the most carefully assembled loads.

Stretching of the case above the web results from either an incorrectly machined barrel or excess residual bore pressure as the case is extracted. I do not have direct experience of the 6mm ARC but simple expansion ratio calculations indicate the timing will need attention. This is not just the size and position of the gas port in the barrel but also the reciprocating mass which includes the impulse from the fire control group.

Regardless, the 6mm ARC is an exceptionally well designed cartridge with an excellent balance between case volume and primer flash volume, which is something that is frequently missed. It benefits from some excellent powders and projectiles.