That is a "punt gun". They were used by commercial duck hunters in the late 19th, early 20th century. They'd mount them on the bow of a boat and slip up on a raft of ducks on the water. One shot, they filled the boat and went to market.
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Actually, the Punt Gun is quite an old firearm and was around for a Long Time before the term Redneck became a fashionable tag.

From Wikipedia
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A punt gun is a type of extremely large shotgun used in the 19th and 20th centuries for shooting large numbers of waterfowl for commercial harvesting operations. Punt guns were usually custom-designed and so varied widely, but could have bore diameters up to 2 inches. A single shot could kill a whole flock of waterfowl resting on the water's surface. They were too big to hold and the recoil so large that they were mounted directly on the punts used for hunting, hence their name. Hunters would maneuver their punts quietly into line and range of the flock using poles or oars to avoid startling them. To improve efficiency hunters could work in fleets of up to around ten punts.

In the United States, this practice depleted stocks of wild waterfowl and by the 1860s most states had banned the practice. The Lacey Act of 1900 banned the transport of wild game across state lines, and the practice of market hunting was outlawed by a series of federal laws in 1918. There are few punt guns remaining.