Classic Army M15A3 - problem with removing the upper receiver

[RostokMcSpoons]

Ok so this gun is a bit of a "blast from the past", but it's been in my loft for probably more than 10 years!

I dragged it out of there with the intention of giving it away as it was DOA, but giving it a last look I found the main problem was a disconnected lead at the motor!  So now I want to attend to the second problem - misfeed / dry-firing on full auto, and maybe I'll be able to skirmish it again.
  
To do that, I need to get to the internals.   I've stripped off the foregrip and stock and removed the two main receiver pins, but the upper receiver won't slide off - it seems to be catching on something and won't move more than a couple of millimetres.
Having looked at old threads on other forums, and some indistinct YT videos, I've ensured the ejection port cover is open, the charging handle is slightly pulled back, but no joy.  It seems there may be an extra pin I need to remove, or some wiring that needs disconnecting, or maybe the piece of silver metal inside the ejection port is catching, but I'm not really sure 

Can anyone with experience of CA guns help?  I don't want to force it and break what might be a very capable second gun

[stryker20000]

There is one small pin you will need to tap out that is around the logo, and also remove the magazine eject button. Once those two are out move the fire selector between safe and semi. You might need to wiggle it out but it should slide out after that. Just watch the wiring for both the motor and battery. Could catch on the shell and damage it. Hope this helps man!