You should look up videos online if you want to do it your self. First go to youtube and type ''How to disassemble m4 v2 Gearbox'' or look for ''mechbox'' on youtube, they have videos which really help.
In Cqb the fps limit is mostly around 350 +/-, I would recommend getting A m90 spring, 20 fps more indoors doesn't really matter, m90 springs put less stress. so why not?
Also you don't need to get a chrono, look up the Pop Can chrono, works real well, look for the video that Airsoftgi made, really helpful.