When this 40mm Burst Cannon came across my Instructables feed, I couldn't resist building one. The cannon is based on 1.25" PVC parts and shoots foam practice golf balls. I couldn't find every part at Home Depot so I improvised a bit. I have a couple of foam balls and will buy more shortly.
I found mine leaks like a sieve from the barrel end and the ball wouldn't shoot without a burst diaphragm (the one in the link does). I don't know if it is due to the dimples on the ball, the size of the ball, or that an electric pump doesn't dive the quick burst that a hand pump does. But, with a diaphragm, it works quite nicely. I tried one disc of aluminum foil (disappointing performance), 2 layers of foil (fun for kids) and 'Duck' brand duct tape (fun for bigger kids). The latter works better and it is easy to rip off square sections to fit. The Instructable only shows discs, but as long as the material covers the internal holed an o-ring in the PVC junction, all is well. Now I need to mount it to a nice base.
I should give the standard warning about the potential danger of pressurized PVC. That is the warning. This thing works a very low pressures. You could go crazy with the diaphragm materials and that may not be true.