That is what was known as a "Punt" gun. Commercial wildfowlers used them to take ducks and geese for the mass markets. They would be mounted on the bow of a small boat (usually a "Punt" hence the name) loaded with a hugeload of heavy shot and the fowler would "scull" (paddle the boat one handed with a short paddle similar to a large ping pong paddle) quietly near or into a flotilla of ducks, geese etc, and set it off killing as many as possible out of a flock while they sat on the water.
JMHO,
BT
Doesn't look big enough to be a punt gun to me.
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