What make is it?
We know the guys at FS quite well and i know they hate with a pssion anything G&G or CA
My mate has had 3 on different occassions, and they all had low fps issues, he sent them to FS to get sorted and they either fixed and broke again or not fixed at all with a note saying couldn't get working, when asked why he was told they're just to tempramental and part sensitive He gave them to me and I fixed all 3 and they still work to this day, I know, cuz i own 2 of them
They are right, they are parts sensitive, but after a couple of hours of matching them good