Originally Posted by Kuro_Neko
There are other problems with gas weapons, especially full auto gas weapons, doubly so in machine pistols. It won't work at all in cool weather. And full auto, plus smallish pistol style mags, means even on warm days, cooldown will be a serious issue. (Cooldown is when rapid firing causes the mag to go cold due to the rapid decrease in gas pressure, which in turn causes the remaining gas to contract, causing further pressure losses and thus reducing the power of your shots.)
If you have your heart set on it, then that's that. But I highly advise against it as a primary weapon.
The KWA MP9 on the market now is the system 7/NS2 gas system which is extremely gas efficient. I have the ASG-KWA MP9 (rebranded KWA) but haven't tried it in cold yet. It does shoot hot especially with the angry gun kit!
My only problem with it is that on full auto, the mag emptied instantly.