Have you thought about a shotgun with a two-barrel set? It seems that you are interested pumps or semi-autos. However, there are some makers, Franchi for instance, that make (or did make) an O/U that could be used with either 20 ga or 12 ga barrels. Check out gunsamerica, gunbroker, auctionarms and search for Franchi. Perhaps a two-barrel set is suitable.
