yea, i have heard some stories about SRCs being.... rough.

You might want to look into reliability over FPS at this point.

I got a Gen III SRC hk416, and the stock internals could easily take a m150 spring...(really good gun btw...)

But as for the Gen II I'm not so sure....even for a m120....

Well heres the specs of it if it helps....

Metal receiver outer barrel; reinforced metal gear box; steel gears and bushings; ultra-torque motor; M110 spring; ported piston head; metal hop-up;

There are a lot of things you can do to increase your velocity without resorting to a spring upgrade. By adding a bearing spring guide, a better airseal nozzle, a better hopup rubber and a tightbore barrel, you should be in the 380 fps range, and that's pretty damn good.

In my C7A2, a Prometheus MS110SP spring alone in the stock CA mechbox gave me 350-360 fps. Upgrading those other components and adding a tightbore brought me up to a consistent 380-385 fps. You should consider upgrading those components first.

Just think price wise for a second then...

What am I comparing too?

If you go from an M110 to an M120 without any other upgrade, you probably wont see a noticable differance...

And i know i have already said this, but untill you've looked at tightbore, new hop-up, maybe a spring guide or a new airseal, a new spring won't really help you

you have a GEN II src then.... And I wouldnt go for the m120 spring.... Like the others said... Get a tighbore and a better airseal and better internal (ball bearing spring guide.... New piston....)
the m120 will just destroy your gearbox...... Or at least cause some premature wear to it.....

An M120 defanitly wont destroy you mechbox, or at least it shouldn't.

It just wont help you :P

My m120 spring puts me around 425 FPS. It used to put me at 435 FPS until I left the spring compressed for a week inside the gearbox.

Remember, the m in front stands for meters per second which should technically be 393 FPS. That is only approximate of course based on the usual standard parts, the rest having to deal with the cylinder/piston porting arrangement, quality of nozzle, bearings in the spring guide AND the piston head assembly, piston head type, hop-up, tightbore barrel, etc. and most importantly the air seal.