It does look like a smaller version of a punt gun. Photo too small for my eyes. I remember an old timer on the St. Lawrence River telling me about them. They did mount them in a boat and used shot, nails, anything they could find. It was black powder. After the waterfowl came in and settled, they would make a noise to get the birds to lift off the water, then fire. He told me he got as much as 27 birds with one shot. I recall seeing only one in a boat house many years ago and I think it weighed over 45#. Obviously, the one the guy is holding isn't that big, but impressive none the less.
The Hermit.