Punt Gun. Used in duck hunting on and around the Chesapeake Bay when Baltimore and DC had a huge appetite for duck before conservation was a thing. Gun was placed longitudinally in a punt, or small boat with the muzzle end in the bow, stock in the transom and the gun was aimed by aiming the boat. Hundreds of ducks could be shot with one discharge. Punt guns were eventually outlawed, and effort was made to confiscate all punt guns known to exist. I remember seeing one during the 1960s on a restaurant wall in Rehobeth Beach DE.

Edit - a great and entertaining read that mentions this gun is James Michener's Chesapeake.