You have a cut-down 1863 Springfield. It might be an 1864, with the solid (not split) barrel band, but they were essentially the same thing. There were thousands of .58 caliber rifle muskets sold as surplus to the civilian market after the War of ‘61. A great many of these were stripped of their sights, shortened, half-stocked, and reamed smooth, i.e. made into single shot muzzleloading shotguns. They are great guns. They are only worth a few hundred dollars now. I’ve seen them sell for $250, and Lodgewood had one last year in pristine condition that sold for $650.
People don’t really appreciate them. If it’s in shootable condition, it will be a lot of fun for small game and bird hunting or just casual shooting. The antique Springfield locks are better than the reproductions in every way.
Good find!
Notchy Bob