Potato gun
the operator will put a projectile into the barrel. A ramrod is often used to push it all the way down to the end of the barrel. The projectile must fit tightly into the barrel so that the pressure of the expoding fuel is strong enough to move it. This is why potatoes are often used. With a large potato, the unneeded part of potato would be cut off when the potato is forced into the barrel. This creates a projectile that is almost the exact size as the inside of the barrel.