a mate just replaced his clear walmart m4 gear box with the jg ver 2 metal gearbox, he said it just dropped right in. he wants to make a "sleeper", as almost everyone we play with has the walmart m4, so now his m4 is shooting about 350ish FPS instead of 270ish, i said he was crazy - but he did it anyway.
so it'll be trial and error - if you want to spend the 100$ on it and try, go for it...
I smell so much bulshit in that statement. Please show pictures of that drop in mechcbox into a walmart gun.