I am envious. I'd call it a Napoleon. Look down the bore. Like Jim asked, how far is it bored, and is there a powder chamber of smaller diameter?
Are there any markings at all? Do you think it's modern or old (condiiton will tell a lot)?
Without a liner there are genuine risks in firing it. You might consider having it lined - that would give you a choice of caliber as well. A good liner could bring it down to golf-ball caliber. Then all you have to do is walk the golf course at the edges of the fairways every Sunday evening and pickup your limit.