I've seen quite a few of these, living where I do and being involved so much in shooting.
I was present of the firing of one of these during the waterfowl Festival in Easton some years ago. It was a cannon, not a shoulder operated firearm. IIRC, the charge was a coffee cup of coarse black powder and the same volume of #2 shot. Damage on the target, a wall sixed cardboard sheet with goose drawings, was impressive.