m120 on 6mm berings?
 

I have a systema complete gearbox with a m100, its shoots very low fps ( astounding ROF however) im wondering if putting it at around 350 fps is ok with the stock berings. Also whats a good spring for 350 fps?

You mean bushings? It's always a good idea to put in metal bushings if you're doing any kind of spring upgrade. Systema completes don't come with metal bushings???
And an M100 should be pushing 350+, close to 365.

Bushings are what come stock...no moving parts...

Bearings what bushings are called when they have ball bearings in them.

Bearings:
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Bushings
http://www.toledoairsoft.com/store/images/p058.gif

And bearings are more likely to fuck up, I think they are mainly used for low fps but high rate of fire guns. I hate bearings, they jam up and area a bitch to fix. Bushings all the way no matter what.

6mm ball bearings wont last long with M120 rated springs. Use solid bushings for at least spur and sector gear.