Just a thought, although it could be a 54 Caliber Smooth Bore double rifle, is it possible it is instead a 24 Gauge Shotgun. The diameter of the 24 Ga is aproximately 0.5787402, which actually equates to a 58 caliber rifle which is the diameter of the roundballs that I use to use in my .58 ca. Hawkins rifle. Being a smooth bore you could use either shot or ball.
Also, unless it is just the angle, it appears the left barrel is out of round, like it has hit something. If this is true you would want this corrected if you planned to shoot it.
Final thought, have you checked to see if either barrel has a load in it. Over the years I have found a number of muzzleloaders with charges in the barrel. Back in the day folks left guns loaded in case they were needed. If your not sure how to check for a load let me know.
Just some thought....
WBLDon
