[audi_bhoy]

Try pushing the mag forward when it's full to see if it feeds. The spring tension in the mag might be enought to disengage the lip that keeps the BBs in the mag.

If you feel the trigger slider passing over the trigger when trying to fire, probably the slider's spring is bent and might just need some plier's love.

Edit:

The cut-off lever hits the trigger slider to disengage power once the gear had reached the specific position, and so the piston stays in neutral position. The selector plates, in semi, does nothing except if you have metal contacts on it's side, it just makes contact then. In auto mode, the selector prevents the cut-off lever to disengage the slider, and so the motor keeps running on and on.