That appears to be a wall-gun.(even though there is no pintle) There were many similar to this - only muzzleloaders and flintlock- English guns, not rifled persussion guns and certainly not breech loading. There were breech loading flinters, and even breech loading matchlocks, predating this 'current' device.

Defending city walls would be my guess for a purpose. The crudeness of this 'rifle' speaks of military intent.

The lock Mechanism puts it post 1825, probably well past 1840 even.

Belted ball or 'winged' bullet - we may never know. Note the 'breech' also has grooves for the belt or wings. I could easily visualize one of Forsyth's explosive 'shell's form this 'piece', blowing up an ammo wagon at long range with this rifle. The 'post' for a peep sight along with set triggers speaks of a long range purpose for this gun. It would be interesting to know the rate of twist in this piece.