I would suggest looking in the Tippmann TiPX internals (I don't know how much you already play/know, so I'm going into super detail mode!). The internals are almost stupid simple and you can do, if you put your mind to it, nearly anything. Plus, they don't require the original shell to make them work, if you want to make your own shell. The best part? The TiPX is made to run on CO2 if you don't want to drop the money on an HPA/Nitro tank yet
. My 2 cents, do with it what you will.