Harpoon gun
A harpoon gun was a tool used in the whaling industry. The gun was a cannon mounted onto the bow of a whaling ship. The barrel of the cannon could swivel to either side of the ship. Two individuals were usually assigned to this gun. The loader would manually feed a harpoon into the gun, while a gunner would eyeball the target and fire the harpoon into the whale. 

A whaling ship that encountered humpback whales George and Gracie in 1986 was equipped with such a gun. ()

External link 
 
Projectile weapons