@superhog: Myself and another user (p0nch3) have been trouble shooting as to why the highcapas suck so bad on GG. Seems to be that a small adjustment needs to be made on the knocker valve.
After the adjustment you can get a full mag of it, though due to the CO2 upgrades it doesn't have enough force to lock back the slide.
http://www.airsoftcanada.com/showthread.php?t=165234