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I don't know, but I will suggest that the difference in the diameter of magazine tube necessary for a 12 gauge from that of a .243Win is cause to not get your hopes up. But Boyds Gunstocks lists some options for the 7600.
if it's just the butt stock you need, there is one listed in the EE parts and accessories for hunting and sporting arms, and I have a very nice walnut one available.
As mentioned the 870 "small frame" stocks will fit the 760/7600 rifles...the downside is that they have too much drop at comb and heel and don't allow for very good cheek weld when aiming.