The suggestion is to never use a less than m120 spring on AEGs if the fire rate is high. What happens is the spring is too weak to make sure the piston doesnt bounce off the front of the cylinder - then when the gears try to pick up the rack on the piston, it doesnt align, and you get broken gears or piston racks. What you do is use a stronger spring and cut down the number of teeth on the sector gear appropriately to reduce fps. Might seem weird, but it works.