This is only a guess, but that ring on the bottom. ... I think you pull down on it and then rotate the barrels. I am guessing mind you.
Speaking of strange weapons (now this is not black powder but was darn close) and this is why I suspect that ring...
I have my Great Grandfather's first ever store purchased modern super duper Sear & Robuck .22 caliber single shot rifle. Its brand is Proven Favorite from what I can read. It has a ring on the bottom of it. You take that ring down/off and the barrel turns counter clockwise and comes out of the receiver, you then load the next shell into the back end of the barrel, put the barrel back in, turn it clockwise until it locks in place, put the ring back and your ready to shoot again. Not what you would call a speed rifle. But its cool.
When it was given to me (My father gave it to me when I was a child. He'd got it from Great Grandpa who from what I heard listening to the old folks, wanted to pass something .. if you know what I mean. He bragged to my Dad as the story goes ... he paid $4.00 for that rifle back then, brand new out of a catalog and that was two days work) GGP told my Dad who of course told me, never to shoot it. Well guess what.. it shoots fine. I cleaned it all up and shot .22 shorts out of it because I was not sure of the integrity of the barrel. For a small front sight and terrible back site, it shot rather well.
The stock is busted and held together with some 100 year old looking baling wire. Seems they were butcher and GGP shot the hog but it did not die, so he smacked it with the butt of the rifle... Rifle broke. Back then baling wire could fix anything.