Theres prolly a very good reason why not.. but..

If you can use a pump to pump up a precharged rifle, or some sort of rifle
(see I'm new to all this)

Why isnt there a special co2 capsule that has a 1 way valve, like on a lighter or something, that basically you could pump up using a pump, then when you put it in your gun the seal would be pressed again and it'd release the pressure.

Ok so it'd be air, and it wouldnt last as long.. but air is free..

*edit* I'm thinking this could be good for us lot with co2 guns for backyard plinking when things like power drop off and pinpoint accuracy arent important.. basically for summer days plinking at tincans etc.. not for hunting.