A close friend of mine has a VFC Scar-L i've worked on. Straight up - fuck that hop up unit. Its that standard clear plastic shit. If you over tighten the screw on the winding wheel you're done for - and you know how easy that is with cheap plastic. Its a shame that you can't buy a metal one.
Im kind of wondering now if the G&G utilizes a similar unit.
Also - watch this video:
https://www.youtube.com/watch?v=gerHLs_Z-6o
He also didn't like how the grip couldn't be changed out either - its very slim.