I have no doubt that the first couple of rounds would probably make it through, provided they have a normal lead core. The barrel would get stressed to a point of failure eventually. I can imagine that steel cased or steel core ammunition would do quite some more damage as lead, and certainly hot lead, is relatively easy to deform.
What I also heard was a shooter using 7,65 para rounds in his unmodified C/96. The gas seal would not be as effectiv as with the 7,63 Mauser round, but otherwise it wouldn't cause much problems. Me, I'd like to use the ammo designed for the gun I shoot it with
