I think it is made for a bullet starter & made to shoot bullets. But unless you find one with a muzzle exactly the same, it is all guessing.
The issue I have with guns like this IF you want to shoot them allot, is it is a bugger to get them really clean inside again. Most have pitted bores & are notorious for holding carbon & etc. Unless you want to spend half a day cleaning it each time, where with a regular flintlock or percussion you could be done in 30 min. You could have a bullet starter made to fit the muzzle.
Allot of it depends on How Deep you want to go with this rifle ($)
So I would study it a while, not be in a hurry, then make a decision. Hang it on the wall or use it. Were I to use it I would remove the barrel & send it to Bobby Hoyt & have him freshen it. That would rid the pitted bore & give you good rifling & good to go. Yes a slightly larger bore but may be worth it to be able to use it. Bobby can tell you Exactly what the twist & type rifling, & most likely can make a bullet starter or false muzzle for it as well, that fits that muzzle. He can do just about anything that involves barrels.