I have a design in my head for this. It's not a potatoe gun, what he wants to do is create something like that motor at the Prescott game, but less power, obviously. Only thing is, it's going to cost a quite a bit of money. You're going to need a tank, a regulator from the tank, a gauge for that part, 8mm tubing would be best, then decide how you're going to shut the flow off once you get your chamber to the right pressure (gauge on the chamber as well), then create a mechanism to open a valve to let the pressure out, while keeping this entire system air tight. I'm thinking a solenoid valve might work, but you might be able to look on Classicairsoft.net for some valve ideas. Might not be the hardest project out there, but definetely not some easy 30 min job. This will take some time, and I would consider this dangerous, so I suggest you have someone who knows what they are doing help you out. If you use too thin of a plastic, pressurize the chamber, and it ruptures, that's going to leave more than just a bruise.