I charge the 12 inch x 1/2 inch injection pipe to 40 psi. It's the most powerful to cosistent ratio i've found. I have fired a 60 psi charge and it blew the potato into tiny pieces as it came out, but it misfires a lot any higher than 40 psi due to incorrect fuel to air ratio. The chamber volume is ...

Here is yet another propane injected spud cannon. It's not anything special but it's simple, sturdy, easy to build, and launches huge potatoes well over 300 feet.