That's a swivel breech muzzle loader - probably built from a kit. These rifles are funny critters - you can spend a hundred hours fitting and filing with files that resemble dental tools (and they are the best thing for the job, no doubt about it! A slip with a dremel on one of these stocks - and it's firewood!). I have a set of 'em called 'rifflers'. Mine are cheap Chinese jobs but they work after a fashion.
Personally I am not enamoured of the concept; but I am a rifleman of sorts, I pick my shots and if I don't like the odds of a clean kill - I am perfectly content to turn my nose up at it and let Bambi walk away. But, from an engineering perspective - this thing is pure genius.
Look at the flawless crown work on the top barrel.
That makes it faster and easier to start the round lead ball and patch
down the bore.
These flawless guns are often built by old retired stubfarts with too much time on their hands, and need something to pour their love and time into. I used to think the best place for such firearms was hanging up on the wall over the fireplace or in the game room... but the old stogies I've heard from that build these things would be incensed. They are meant to be admired AND shot, and even a doe or a spike buck becomes a trophy when harvested with a gun like this.
I was going to build one years ago myself, but life and circumstance got in my way and now it's probably too late to develop the skills required. I might do a pipe hawk and maybe a knife yet... I'll have to see how that goes because I don't have the skill for that either! For now, I need something to keep my hands and mind busy while I suffer through a bout of unemployment and look for my next job(s). We'll see how it goes... if my sanity holds up, I just might get some arts n' crafts in.