I have taken deer with Sierra 225gr GameKings in a .35 Whelen that also vaporized inside a 120 lb whitetail. I presume the bullets were made for the slower velocities of .35 Remington than the .35 Whelen. The jackets and lead were just shredded as the shot was close about 45 yards. Also used the 225 Sierra on deer with .375 H&H with the same results of broken jackets. The 225's were more likely designed for .375 Winchester velocities. This may be the same situation with your Speer's. Just driven too fast.

I like your long bottom tang on the rear trigger guard.

I have a Oberndorf Mauser from 1943 that was a Portuguese contract I am told but never made it there as they were diverted to the German army as it has their acceptance marks all over it.. It was sporterized and has the side panels as yours does, yet still in 8MM. I have come very close to re-barreling to .35 Whelen which would near the performance as your 9.3x57.