Perhaps you can build a scaled down version of a pneumatic spud gun out of ABS and use a gravity powered hopper (use an inverted neck of a Coke bottle as the hopper and then glue gun the cut bottom of the bottle back on it, making sure the neck faces down.) for holding the paintballs. You can buy HPA (high pressured air) tanks at a paintball store. Ask the dealer how it's used, and then buy a very big tank of pressurized air to keep filling it as you run out.
If you choose to do it this way, don't use PVC. It is stronger than ABS, but because it is stronger, it is more brittle and it will just explode under high pressure. ABS is softer and can stretch up to 33%. If you don't trust the plumbing tubes though, register at a spud gun community and ask them for help. I personally recommend Spudfiles.com, I use this website often for many of my builds. In fact they have a guide for BB spud guns. Be sure to check it out!
Good luck, if you need help feel free to contact me (I'm extremely free) and I'll try to help you find the answer.
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Check out my instructable "aerosol powered paintball gun" for a quick and cheap version.
thanks man!! very cool.