Punt guns were enormous shotguns used to hunt waterfowl in the 19th and early 20th Centuries. They were so heavy that they were normally attached to small boats called punts and the boats were then pointed as birds resting on the water’s surface:
Punt guns were usually custom-designed and so varied widely, but could have bore diameters exceeding 2 inches (51 mm) and fire over a pound (0.5 kilos) of shot at a time.
A single shot could kill over 50 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.
Generally the gun was fixed to the punt; thus the hunter would maneuver the entire boat in order to aim the gun. The guns were sufficiently powerful, and the punts themselves sufficiently small, that firing the gun often propelled the punt backwards several inches or more. To improve efficiency, hunters could work in fleets of up to around ten punts.
The practice faded as wild waterfowl stocks were depleted. It was eventually banned in the United States, though I gather it is still legal in the United Kingdom.