I think you have a Zulu.
It started out as a Belgian breechloading conversion of a
French or Belgian military rifle musket, as did the American Trapdoor or even closer to the British Snider, which opened sideways like that. After it was obsoleted by repeaters, it was reamed smoothbore and cut down into an inexpensive shotgun. It was cheap then, it does not have a lot of resale value now, there was one on GunsAmerica going begging for $225. Hang it on the wall and make up stories about it.