my bad, i thought the gng was the one with the long hop up and long nozzle. Thats why im going echo1, the gearbox is a lil longer at the front, rest of the parts are standard v2 afaik
edit: evike says standard m4 piston (guessing they mean v2 lol)
Gearbox:*G&G Gr25 series uses regular M4 type piston. Not the long type.
http://www.evike.com/products/28222/
not that i have huge faith in evike, best bet would be to count the teeth before buyin a piston