CO2 Mags in Hong Kong has lower airflow plus 20g cartage has limited space to contain gas may not finish a mag in high powder setting.
If you don't know what you are doing with GBB, then GHK or WE (open system) is good, but both has lower J (FPS). But you should really looking at replacement parts, GHK's is hard to find parts, WE has lots of parts and lots of so call (up grade parts), both needs experience to install and modified "up grade" get it working and powder up.
If you looking a M4 style, WA systems has too many company, you need lot more money to try different parts to make it work, the most important in WA systems is the MAG!!!! b/c air control is at the MAG.
M4, you should look at VFC or KSC those system have solve air control problems, right now there are no up grade parts, but those gun shoot around 1.5-1.7 J out of the box and Hop up is very OK. VFC / Socom Gear parts are very reasonable a bolt carrier with loading nozzle is under $200, Mags is around $200. KSC is very good (Taiwan version), but can't find Mag now, have to wait
