sounds like a burnt out fuse to me but if you want to open check the gearbox...
to get the gearbox out of a m4/sr16/m16a2:
1) use the break open feature and seperate the top and bottome halves competely. if its your first time using the break open feature it will have some resistence, when i first tried it i though i would break the gun too.
2) take the stock off. first take off the sliding part by retracing it all the way out, then remove the small screw located in the back of the buffer tube, its on the bottom side. this will allow you to remove the rear cap of the buffer tube and inside the tube will be the last screw that holds the stock on.
3) remove the motor grip plate. there are two screws on the botom of the pistol grip, take those off but dont touch the small grub screw in the middle. when you take the motor plate off be careful not to lose the washer thats inside. note the wiring placement, unplug motor and remove it.
4) look inside the grip, you will see two screws, take them out.
5) remove mag release assembly. theres only one screw here so its easy, its a small allen type screw located on the face of the mag release button.
6) take out the small pin on the lower body, its right next to the fire selector. use a small pin or needle to pus it out, it will come out easily.
7) remove the feed tube that is right under to the air nozzle. theres two screws here, lift it out gently.
put the fire selector in halfway between safe and semi, and pull the gearbox out from the top.
Wolf, that strange little button is called the forward assist.
