There's some discussion of CO2 cannons over at
http://www.spudfiles.com/forums/viewtopic.php?t=308&
Whatever you do, make sure all components are rated for the potential pressures involved at a particular stage, which can be up to about 2,000 PSI with CO2. Do incorporate a quality regulator and an overpressure relief device in the charging chamber. I would suggest using steel or aluminum tube for the barrel and avoiding any PVC for safety reasons.