Pneumatic Potato Cannon

A rubber diaphragm valve is used in both cannons. It’s made from a standard 4″ rubber end cap. The ones I found have a convenient concave end. After cutting off the part of the end cap that is supposed to fit around the outside of the tube, the part that covers the tube end fits inside a standard PVC end cap.

Pneumatic Diaphragm Valve Animation

When there is no pressure inside the gun the curved diaphragm doesn’t make a very good seal with the end of the barrel. When the gun is pressurized air leaks through a small hole in the diaphragm and fills the space behind it. Because there is much more surface area with pressure on it behind the diaphragm than in front it is now sealed against the end of the barrel. When this pressure is released there is no longer anything holding the diaphragm to the end of the barrel so it snaps back allowing high pressure air to enter the end of the barrel.