Capt. Jas. gives good advice, especially if you want to be correct for the 18th century. There were some short guns, but getting it right is tricky. Many cut-down guns were done so crudely; other short guns have a completely different barrel profile from a modern shotgun - a blunderbuss, for example.
That said, here's mine - not necessarily historically correct, but fun:
It has a 26" bbl. in 16 ga. "Sweet William" Douglas built it for ChuckPA, and I traded him for it years ago. It "could" be a trade gun, an oddball carbine, or just fun.
