Do I need a spring change?
 

Just got a G&G CM18 Mod1 and the field I play in has a 350 fps limit. Now whats confusing me is that ive read online that this gun comes in at 340-350 fps default but at the same time ive heard its way over that and you need an M90 to play indoors... I dont have a chronograph so I wanna be ready when I get to the field..

Different countries have different FPS standards. If your gun is brand new it will be shooting above 366 FPS because that is a legal requirement for importation into Canada.

If you bought it brand new from a Canadian retailer it likely has been upgraded to 366+ fps to meet requirements for importation. The only way to know for sure is to chrono.

Unfortunately, I don't think the G&G CM series comes with a mechbox with quick change spring capability so you're going to have to get an M90 or M100 spring and read up on taking the mechbox apart or have a gun tech do it for you.

I bought it from hero outdoors

do a can test for an estimate. or go to an airsoft store, they all have chrono

I will, thanks