Just some thoughts:
Light charge and S. E. E.
Last warm up load might of been a squib load, leaving a bullet in the barrel. I have seen this before.
Barrel obstruction. Snow, mud, ice. Maybe a cleaning rod left in the bore.
Wrong powder.
Overcharge
100,000psi? He is lucky the rifle did not turn into shrapnel.
Worst blow up I have seen was a Carcano. Shooting mil-surp ammo, bullets were pulled and replaced with long soft nose. Case mouths were loose. A live round was chambered, then ejected. The bullet stayed in the throat, not noticed. Next round chambered had another bullet ahead of it! Pieces of steel and stock splinters everywhere. The shooter was badly burned over the face.
