I personnaly use CA andGP SR25 magasin to fit in my G&G. You have to cut a little 1mm to the rear of the hole to hold the mag withthe mag cath to fit correcly, but the magasinne are cheaper (about 15$ each) and full metal. I'll add some pic if you request for.
There is a super link to use for G&G SR25 to know wich part are compatible with each other brand:
http://www.airsoftforum.com/board/SR...e-t245076.html