Back when I was working at the gun shop in Georgia, I had a cartridge collection prominently displayed on the shelf behind the counter. Folks were always bringing things in to add to it, and the star was a 4 Bore cartridge, which made even the mighty .50 BMG look a little spindly by comparison.
That is, it was the star, until a guy brought in what looked like a brass-based paper towel tube. It was labeled "0 Gauge", and was the shell from a breechloading punt gun from back in the days of market hunting. You could have dropped the 4 Bore and the .50 BMG down in there, added a .600 Nitro Express and a .458 Win Mag for good measure, and rattled them around like beans in a maraca.
Xavier has video of a sidehammer punt gun being fired.