Big Fun! Everybody needs one of these. My design uses a measured amount of propane for the charge. The small pipe is loaded to a set pressure and then dumped into the combustion chamber at the bottom of the gun. I use an electronic grill ignitor and a remote 25 foot cable to fire it safely.  As potatoes are not the only amunition you can use, I made up several barrels in different diameters. Small stuffed animals make great ammo.