The final version begins with the CO2 coming from a paintball cylinder.  It enters the gun through a check valve (to ensure it does not come out once it enters the tank) and then a Quick Exhaust Valve (QEV).  The tank has a gauge on it to show pressure, a blow off safety valve which releases air over 175psi to ensure nothing bursts (although the system can probably handle up to 300psi).  When the air is fully charged, a blow gun is used to change the direction of flow in the valve to allow the air in the tank to exit through the barrel and hence launching the projectile.
