Now dodger-me has a preety good idea, in paintball i've seen people with a rap 4 pistol using co2 although it's for paintball I belive their one for airsoft. Take a peek guys
http://www.rap4.com/ although this method is old, it works (I think) where you get a co2 bottle and a adapter on the bottle and plug it into the mag and charge her up. I definitly do not know would it work for airsoft mags (I think only WE can do that with cartridges though) so don't quote me on it, it's just an idea
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