Morgan,
Dismounting the barrel should only involve pushing the wedge across from right to left in the foreend, and placing the lock on half cock.
Also removing the ramrod.
The wedge can get stuck though, with rust or age/old oil . Said wedge normally does not come all the way out, as it is often split and has a tiny pin through it, so it cannot be lost.
The bun nut holding the hammer in position would originally have had a slot in it, to take a split turnscrew.
Engraving style I'd put in the 1820's. It Is quite an early one for percussion, and very nice!
Mike, you beat me to it. our posts crossed.