What kind of shotgun are you looking for? bob1965 has a pretty decent selection—for free, no less—over at OnTarget3D...
Win1897 - based on the Winchester Model 97, a slide-action model with 3 different (long, riot and trench) barrels via geometry switching.
Coach Gun - a side-by-side double-barreled, break-action design of the sort Wells Fargo issued to its drivers for defense beginning in the late 1850s, and still manufactured today.
Over and Under Shotgun - another break-action double-barrel design; as the name indicates, the barrels are vertically stacked.
All three feature working actions, triggers, hammers and the like. And did I mention they’re free?