I looked at photos 19 & 21 and from the looks of the damage I would say the it was caused by misaligned live rounds striking the edge of the chamber opening. The bolt face and the breech face are mated machined surfaces and unless there were something foreign on the face of the bolt or the breech this damage is not possible. The sharp, hard nose of a .50 BMG will act as a perfect punch if it is slightly out of alignment with the bore of the weapon. I think this is referred to as a "jam" or "misfeed".
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