[Hectic]

Sounds like a broken/worn cutoff lever.
basically that lil lever u see on the outside of the box has a portion inside the gearbox.
That little portion rides on a cam (a lil teardrop shapped bit on one of the gear shafts) once it(the lever) hits the point/tip of the cam it slightly lifts the shuttle (the moving bit of the trigger contacts) off the trigger and the return spring pops it back to battery.
If the lever is broken or worn out the gun will just shoot auto on semi.
it could be the trigger shuttle itself but if that breaks/wears youll normally just get a click sound as the shuttle pops off the trigger so prolly its the cut off lever.