Guns larger than 10 ga are generally forbidden for sport hunting in the U.S. At one time larger (much larger) shotguns were in use. Called "punt" guns, market hunters used them for ducks and goose. They were often mounted on a pedistal in the boat, though sometimes a hay bale was used between the shooter and the gun.

My best friend when I was a child was an old man, Captain Mex, who told me tales of hunting when he was young. Mr. Hogge (to us children) spoke of firing the gun at first light, and gathering ducks until lunch. Although the stories are interesting, they are also sad, as market hunters almost drove many species to extinction, and did some.

The Virginia Dept. of Game and Inland Fisheries has a nice collection of punt guns in their HQ in Richmond. It is rumored that a few are still hunting on the Eastern Shore of Chesapeake Bay, in Virginia and Maryland.