Shot loads, eh? You'd probably want a smoothbore then. Most any single shot black powder pistols you come across are going to be rifled. I have an old flintlock 14 gauge made in Japan for Dixie Gun Works in the 70's. I can shoot 1 ounce birdshot loads, buckshot loads, or even big .690 roundballs. It isn't very accurate or powerful, or dependable, but it can be fun to shoot. It isn't a hunting arm or a target arm though, so if I hadn't gotten it for free I probably wouldn't have it now.
A lot of the Dixie gun works kits for derringer kits are smooth bored. The Duckfoot pstol, the pepperbox, and a few of the derringers are smooth bore. Most of them have a .36 bore though, and that would be a pretty tiny shot charge. Most of the kits are under $100 though. It might be worth a try.