NOw that yo mention it, it reminds me of something between spring and gas.
My CA M24 is a spring gun, 150SP spring in it right now. Spread between my 0.20g and 0.43g is what, roughly 130fps? 464fps (0.20g) vs. 337fps (0.43g). After I installed a tightbore in my KSC Glock 19 and using propane on a warm day, after chronying my MP5 I cleared out some of the 0.43g BBs I had loaded in there for the game and put a couple 0.20g so I could see what it was shooting at. Both shots came up as 341fps using the 0.20g, but the next round was a 0.43g and it came out at 265fps. Heavier BBs stay in the barrel onger, allowing the gas to expand more. This kinda was another example of something Illusion was saying about heavier BBs in gas guns not losing as much fps compared to spring guns. So instead of losing the 130fps as in the spring gun, I lost only 75fps or so in my G19.
I also have a KJW M700 gas sniper rifle that I'll be running through some tests as well, but since it's gas powered and has an adjustable valve, it's rather pointless to post up anything about it since every gas rifle is different.