Is it OK to leave FX magazines loaded?

Leave them loaded. no problem.
Its a fairly long spring, it have tension all the way to the last pellet. so its always under an amount of load.
I pretty much load the magazine when finished using it, and stuff a full mag in the FX next time im shooting, dont want to have to load the mag if I see a opportunity to use the FX unexpectedly. have not had any problems.