Benelli M4, Marine configuration (Full length tube, accessory rails, ghost ring night sights, collapsible stock)
I think I'd do alright. 12 gauge is a common round, and the shotgun's worth a lot of money, I could trade it for something a lot more practical pretty easily I believe.
It was either that or the Beretta M9, which I would be better off with from the beginning: Could carry more ammunition that happens to be even MORE common than 12 gauge, faster reloads, lots of spare parts...