Have you ever heard of a punt gun?
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 exceeding 2 inches and fire over a pound (.5 kilos) of shot at a time.
2 inches is over 50mm!
20mm is considered a cannon in the military! Unfortunately not many punt guns exist these days and most are not capable of being fired.
Here are some photos of punt guns and two videos showing one in operation.