It's a true 11 gauge, I'm guessing, because that was a very common bore size in Great Britain in those days. 11 ga. is .75 calibre, the bore size of the famous "Brown Bess" muskets. Gunsmiths would've been quite used to turning out barrels in that caliber.

This gun, in such great shape, would be very safe to shoot with black-powder loads. (Of course you shouldn't use smokeless powder in even modern reproduction muzzle-loaders!)