Excellent picture! Sometime during the Civil War, I assume. I like the base of the mortar; looks like it swivels a full 360 degrees.
A guess on my part, but in a smooth-bore mortar, wouldn't the "base" which the ball is setting on be attached so as to form a gas seal? I'd think that in such a short barrel, without something acting as a "wad", there would be a lot of gas blow-by. Me not do dem backpowder stuff much.
Sincerely,
Michbob
PS: Useless piece of trivia: one masted ship with siege mortar: a "bomb ketch".