You may find this thread to be of interest: MS 8mm

By the way, please do be nice to that rifle. It is among the finest bolt action sporting arms ever produced. When / if disassembling anything on that rifle be sure to use proper (thin) drivers. Please excuse me if you already know this, but it is important. Also, if it is full stocked (stutzen), the forend tip hardware is very delicate. Use great care if disassembling / reassembling

As MS rifles and carbines (stutzen) have been offered in a variety of chamberings through several decades, many (too many) have been altered over time, and there are a plethora of potential bores / rebores that would be proofed as '8mm", I cannot overstress how right Casper's comment is regarding chamber cast - do it.

No one wants to damage a fine old MS (or even a 'sporterized' Greek military MS) and ' or its shooter when caution and prudence could produce a better result.