Large caliber handgun, usually magnum catridges, such as .357 Magnum, .44 Magnum, .50 Action Express, .454 Casull. Also, commonly in reference to revolvers.
"Marvin, why didn't you tell us there was a guy in the bathroom with a hand cannon?!?" - Vince, Pulp Fiction
A hand cannon was the mother of all fire arms . It was simply a metal pipe that had gun powder put down it and a rock , or lead ball. It was fired by a piece of flaming rope put down in a open hole.
Short-barrel shotgun (Around 10-12") with a pistol grip. Usually made from cheap breech-load single or double barrelshotguns. (Over-Under models are also used.)