To use compressed air in a QB, you would need a regulator to give a steady 800 or 900 psi, otherwise the breech pressure and muzzle velocity would drop with each shot. Also, if charging to "only" 1500psi, you would only get a few shots before power dropped off even with a regulator. With CO2, as each shot is taken, more liquid boils off to maintain pressure, until all liquid is gone. (OK the gun chills and the pressure drops a little with rapid fire). For all the hassle of charging gear and frequent topping up with air, I would say it's better to stick to CO2; simply find a source to bulk buy the 12gram caplets. In the UK, Walther caplets from many small shops are $1.70 each as singles, but we have mail order sources where 100 caplets is only $80. To me that is economical shooting, 80cents for 30 to 50 shots, depending on tune. There must be other sources of caplets in the States than Crosman. Think of all the tyre inflators, fire extinguishers etc..