Actually, any smoothbore is a shotgun. A fowler, Tradegun, Musket, whatever, it can fire shot loads and be used as a shotgun. The Pedersoli Bess carbine has a 30 1/2' barrel, handles well, and is an 11 ga. Should work for what you need.
I didn't see any larger bores though, or did I miss them? It's been a long week, a very long week, maybe I'm just not on the ball.