Wiktionary
spud gun
n. 1 a toy gun that fires a plug of potato by compressing the air in the barrel, forcing the potato 'bullet' out at speed. 2 a larger version of the same, powered by compressed air, butane or other propellant which fires a complete potato
Wikipedia
Spud gun
A spud gun is a small children's toy gun used to fire a fragment of potato. To operate, one punctures the surface of a potato with the gun's hollow tip and pries out a small pellet which fits snugly in the muzzle. Squeezing the grip causes a small build-up of air pressure inside the toy which propels the projectile. The devices are usually short-range and low-powered.