Are two size HPA regulated air bottles that will fit, the 13 CI and the 22 CI. tyhese are little tanks with regulators on them. made for paintball, but will fill to 3,000PSI and are regualted to only output lower pressures (and i'd stick with 850psi). Might exceed the 12 foot pound British limit as air is "thinner" and flows more easily than co2, so usually gain 2- 5% in speed (with the tank regualted to 850psi)
Never a good idea to fill a co2 tank with anything but co2, but probably no danger if you don't exceed co2 pressures. Won't find a whole lot of consistent shots that way, but it would work for a dozen reasonably consistent full power shots or so. Am reasonably SURE you'd go over 12 foot pounds.
