Vince is Right, Most companies including tm (although less tm then others) have a hard time getting shimming right. Usually they overshim to the point where the gun has a hard time pulling an M120 w/out some serious work.
As a note for the CA M14. I'm running an M120 set up in mine with bearing spring guide, Systema M120 and a Systema Magnum Motor, as far as my recomendations, They are:
Reshim
Regrease
and use the CA pinion gear if you change motors.
I run a 3500 mah 8.4v NiMh and my god its got an awesome trigger response (there is a trigger response?) Lets give an example. Its very hard for me to do a fast double tap w/out the gun locking up and blowing a fuse. I cant tell if this is because of the way the motor is or my own stupidity. Granted, an easy fix but the Magnum has enough torque that if you double tap in the general sense it WILL catch the piston mid cycle and lock the gun up. The only solution is modifying the gun to shoot 600 fps on semi only, but I feel that might be accessive for Airsoft Arizona games (I'm a trustworty person, you wont see me shooting anyone up close
) and I will refrain untill at least I feel comfortable with that set up. Right now, 400 fps with no trigger lag, is awesome.
I'm in love with this M14, I liked my TM alot but the CA just has won my heart, its everything I wanted out of my TM and more, even if it is an inferior copy of an inferior design.
Its also got standard CA internals, so I would imagine you can just drop an M120 spring and a bearing spring guide in there and you'd be set at 400 fps. Maybe a reshim and regrease too.
The shimming was very tight on mine, so tight it couldnt pull an M120 spring with the stock motor (hence the Magnum) by the time I got the magnum I had already stripped down the gearbox and done what i'm not beginning to do with every stock gun purchase, tear it down, reshim it and regrease it.
Vince is right though, if upgrades are done right they actually INCREASE the life of a gun as apposed to decrease it.