Nubie Gas Question
 

I'm new to the world of airsoft and I'm really curious about the types of gas that is used to power airsoft guns.

Paintball and airgunners have been known to use bottled CO2 but I don't see it used in airsoft. Instead I see mention of "green gas" or propane.
Why not use CO2? Is it because of the pressure levels?

Because most pistols are built to use propane (the magazine is the pressure vessel), not CO2 caplets. And yes higher pressure means higher FPS in general, and sidearms are usually used in CQB or closer ranges.

Also paintball's are .68 grams and large, bbs are smaller and require much less energy to get them to leave the barrel. Tippman did release a propane powered marker but it was a spectacular fail as it had no shot to shot consistancy. As Styrak pointed out pistols are mainly used for cqb where close engagments are the norm. As we play with friends we generally don't try to injure one another.

Paintballs are commonly .68 caliber not .68 grams. They are much heavier than that.

I have one of the markers you mention, the C3. It doesn't use the expansion of propane, but rather the combustion of it to propel the paintball. And actually, once it 'warmed up' (literally) it shot pretty good. I hear the reason they didn't move forward with the idea into semi automatic is that there was excessive heat build up, from the combustion. Too bad as I think it's a really cool idea.