Here's one way...
http://www.youtube.com/watch?v=f5jyd7H4JW0
Hopefully Youtube will offer up the related videos... we were inspired in our design by 1726. (Still grateful for that, guys, even six years later.) If not, this thread might have more information
http://www.chiefdelphi.com/forums/sh...ht=1726+launch
Okay, I know you mean "use compressed air like a potato gun", but while there are ways to do that, I don't think many of them are FRC legal, and those that are legal might not be particularly practical. But you CAN launch a much larger, heavier ball using compressed air.
Jason