M4 wont fire? help!
 

I bought a G&G gr16 plastica a few days ago. I charged my 8.4v 1100mah ni/mh battery fully and plugged it into the gun, loaded some rounds into the magazine and fired away. After 3 shots the gun stopped firing and every time I pull the trigger it makes a sound like the battery is dead or a the fuse is blown. the gears only move the slightest bit. The fuse is fine and the battery is fully charged. I opened it up and all I could see was that the piston head was fully compressed and it wont move. Any help please?!

Yeah, I have seen many of those problems before.
Your battery doesnet have enough voltage. Get a 9.6v battery.

An 8.4v is more than enough to fire a stock G&G gun. The problem is more likely a locked mechbox resulting from incomplete trigger technique.

First, put it in full auto and see if it fires. Otherwise you'll have to pull the motor off and trip the anti-reversal latch.

Mechbox lock occurs when you don't fully pull the trigger and let it forward before the mechbox completes it's cycle. You cut power to the motor, so it dies with the spring close to fully compressed.

How do I trip the anti-reversal latch?

Did you try putting it in full auto and shooting a few rounds? That happens to me sometimes and shooting in auto fixes it.

I don't know exactly for an M4 (never owned one). I know you have to remove the motor from the mechbox, then you can reach in with a pair of pliars and pull the latch up to unwind the mechbox.

Remove the motor plate and pistol grip. From the bottom of the mechbox (rear), get a small pliers or bend an strong wire and hook it on the ARL and pull it. This will do the job if your mechbox is jammed.