Co2 expands pleanty fast for an RT. it's the fact that it's pressure changes so much with temprature. CO2 will OPERATE an RT.. but you'll end up with velocity spikes in teh +-200fps range. AKa, blowing holes in paint, or the ball not making it out of the barrel. Nitrogen is required because it's a fairly temprature stable gas. The gun recharges so fast, that it even heats up n2 enough to change the velocity signifigantly!
You'd have to be Crazy to be sane in this world. - Nero
Omnipotence ain't all it's cracked up to be.......