I built one while in the Air Force and instead of potatoes I used a piece of high density foam wrapped in duct tape. It kind of looked like a big tater tot. I would lube the barrel with G.I. spray lubricant and used Aqua Net for the propellant. We would shoot them across the steam plant where I worked at the vinyl welding screens. The boss wanted to know where all the holes came from. I got rid of the gun, I had used a push button barbeque ignitor and it shocked your hand everytime you fired it. Fun but dangerous.