It is gauge stamped 22 between the proof marks. It is an original trade gun of the type intended for African trade, not a reproduction. Late 19th century. Note the use of surplus Enfield parts - bridle, sear spring and likely the sear. It is in remarkable condition for this sort of gun.
Don't know what tools you have at hand. I generally pull loads, rather than unbreeching the barrel. There is a risk of damage. Assume that it is actually loaded with powder/shot/ball/whatever, but be prepared for anything. All sorts of things got packed into muzzleloader barrels. Just unloaded on barrel of a double - brown paper wadding, and a round ball, but no powder. No idea if it was a dry load or someone playing. Unloaded a Barnett Northwest gun once, it had been loaded as a shotgun with newspaper wadding and #5 shot. Newspaper was from the Winnipeg Free Press, Nov. 13, 1924.