" The only ones that I know of that have shot off the face are the Merkles in 416 Rigby and 375 H&H, and that doesn't surprise me, those rounds are too intense for a double rifle action "

Have you ever seen one off the face, or just heared of one? I have a Merkel .375 with hundres and hundres of rounds through it and it's as tight as the day I bought it. I would venture to say it would take thousands of rounds to bring it off face.