[Zekk05]

So heres the situation.

I started building a C7A2(A3) a while ago, and put a systema mechbox in.
put in the motor, tightened it properly, pulled trigger and it shoots. great. go to chrono... 270fps.
huh?
as far as I know, m120 springs are supposed to be shooting around... 390!
Either way, the gun had sadistic range, so even with the low FPS I was able to play with it for a few games without issue ... until the red piston broke! score.


A month or two ago, I snatched up another M120 mechbox off of here for a very good price. Got around to fixing it last night with my tech, and it works flawlessly... So, slap it in the gun, pull trigger. and it shoots. Great. Go to chrono... 275 fps. DAMMIT!

The first mechbox was used in conjunction with a guarder hop up and deepfire 6 04 barrel.
The 2nd mechbox was used with a systema mechbox, and systema 6 04 barrel.

Is the 120 spring only rated for 270fps? We put 120 springs in some JG rifles and they fired 420 fps!
Compression is damn near perfect. Cylinder head isnt damaged, nor is the air nozzle (both brand new) hop up is off. what could be goin on here?