[ShelledPants]

This happens from time to time because of an issue with the bolt and it's alignment with the trigger group. If the bolt is marginally to the rear due to a chopped bb being stuck in some obscene place around the nozzle or even above the bolt, the gun will appear to be held together by magic even with both pins popped.

What you have to do is (with some patience):

Rack the gun and set it to semi. Close the bolt. The hammer is now cocked.
Pop both receiver pins, rear first.
(and here's the hard part to conceptualize)
While rocking the bolt back and forth 1mm-2mm, attempt to move the upper receiver directly upwards and forwards.

This motion is NOT the same as pivoting the receiver around the front receiver pin. Just straight up from the lower and forward along the path of the barrel.

If you CANNOT get the receiver to split, attempt the above with the gun in the fired state, with the hammer decocked.

I've had to fix this on a buddies gun a number of times while he was getting his Open Bolt kit worked in on his old as fuck Closed Bolt gun.

If you get the monster apart, check EVERYWHERE on and in the bolt for a chopped BB or loose / broken parts before reassembly. Also check the trigger group visually and the topside of the upper receiver and hop up unit opening. White chips in any of these places will cause the weirdest shit to happen.