The cannon is one of the most powerful weapons in the game, using pre-loaded explosives bathed in blood. As such, it requires no ammunition to fire. The five projectiles it launches when fired cause considerable damage to anyone near the impact point, including the player if they're not careful. Note that it prevents dodging when equipped, so use wisely.
Los Bridge East, found in footlocker on hidden island. The player must leap into the dark void just east of the last saving terminal near the exit from the level. Instead of dying, they will land on solid ground. From there, the player has to continue on in a straight line to a destroyed bridge. After wiping out the ghouls, the meat cannon is in the footlocker toward the ice cream truck.