Dunno quite what you mean by this:
"A case may fail at 70,000 psi"
Without the chamber-walls, I presume a case would fail at 1000 psi!
Case failure, in the absence of any rifle failure, would be markedly affected by breech design, extractor type & fit esp. width of extractor cut, etc.

In Musgrave's case, they may have 'proved' the action during the design phase, using very stiff measured proof loads and a rigorous scientific measurement process, and simply prove each individual rifle for function as a formality, or perhaps for legal reasons.