I can not help you with "what is it", but some clues - most military mausers had stepped barrels - that one is either machined down to be tapered, or has had the barrel replaced. A "not usual" side scope mount -would be interesting to find the rest of it. Receiver looks to be a full on military mauser 98 of some sort. Cool pistol grip treatment - not your average "bubba", I don't think? As if the extractor collar is turned or deformed somehow? Should be smooth and even with the bolt body. Do you know for sure what cartridge it is chambered in?? - There are some marks, likely proof marks - on left side of the chamber, I think, but I can not see them well enough to read them. The bolt handle looks looks to have been cut off and then re-welded like it was made to be "scope friendly", but shroud still has the "over the top" safety lever from military - not certain how that would work together?