The core of the idea is to make your own paintball gun for cheap. We also didn't want to be spending money on refilling CO2 canisters. Again with the cheap.
The first part of this instructable is how to build a spud gun style paintball musket. If you're familiar with a spud gun or have already built one, this will be simple for you. There are only a few simple changes to make for paintball.
We did try compressed air but it didn't have the range we wanted. However we were only trying at +/- 100 psi. Some portable compressors say they can go to 300 psi which might work much better.
This isn't an absolutely unique idea obviously. There are others that have done similar projects.
The main idea is to convey how to and then introduce the idea of a different class of paintball contest.