Need some help...with springs & motors
 

Hi guys, here's the situation

I have a friend with a G&P Magpul MOE CQB edition, bought from Revolution. The description of the gun says it comes with a G&P M120 motor.

The gun tore itself to pieces in a matter of days. Mech box shell is fine and gears survived but the piston exploded. When he disassembled the rifle, he discovered a G&P M170 motor, not the 120. He replaced the Piston with an SHS Metal Rack piston with second tooth removed. The gun began firing at a whopping 460 FPS. He put in a Classic Army 120 spring and is still firing over 420 FPS. He is afraid that the M170 motor is going to destroy his new piston.

So, if he were to replace the motor with say a 120 motor, would that solve some problems...or would it create more?? Can you put other brand motors in a G&P M4?? Any suggestions guys??

Thanks

1. The motor has no impact on the resultant FPS of the shot....just how hard/fast the piston gets pulled back (so influces ROF).

2. Looks like a nice tight setup and airseals are good :) Almost all spring ratings (i.e. M120 = 120 M/sec) are based on "old stock mechboxes" that did not have bearing spring guides, bearing piston heads, tightbores, etc...all of which affect the FPS. So, if an M120 spring is still resulting in too high an FPS...then drop down another level. If you stick to the same manufacturer of spring...then the relative differences should be close enough for starting guestimates (i.e. if you want to drop 25-30 fps from where you're at now, then get the M110). It's never a bad thing to go down a bit lower than expected...maximum field limits are MAX numbers. I've always found a really good sweet spot for accuracy and range between high 360's to about 385fps. For indoor stuff you really don't need more than a M90 spring.

3. If you run too fast a motor with too light of a spring you raise the risk of having the gears come around and crash on the piston before it's fully forward. Downgrading to a slower motor will help guard against that. I'm a bit dated on motors now...but I'd just go with a Mauri EG1000, G&P M120 or something in that range. IMO....super big/fast motors just mean higher risks of a catastrophic failure.