[Kos-Mos]

Push the forward assist (it will release the spring).

Remove the rear body pin.
Flip the gun open.
Press the trigger (without battery).
There should be a small plastic part with a spring that moves. If it does not, it's because it's locked at the front. (happens some times).
Release the trigger and use something to push down (towards the inside of the mechbox) on the small block in the center. It should spring back as normal.

If it moves when you open it, it's probably between the fake bolt catch and the delta ring (the little metal guide rail on the left side of the magwell that covers the wires). The wires always are pinched there and get stripped (and short since it's metal). Especially if you pull on your wires to unplug your battery instead of holding the plug.