I'm with Eeyore on this one. Make sure you have a good AEG first. I am just looking into my first GBBR as well. If I didn't have my AEG I wouldn't even be considering running gas of any kind, and by gas I mean I will be running C02. The climate where I live isn't friendly to gas guns so C02 is the best bet, atleast I think so.
"Do not touch anything unnecessarily. Beware of pretty girls in dance halls and parks who may be spies, as well as bicycles, revolvers, uniforms, arms, dead horses, and men lying on roads -- they are not there accidentally."
Soviet infantry manual, issued in the 1930's