CYMA Mp5 - Splitting the front and back receivers

[Bar1975]

hey guys.. i have a full metal mp5 and want to tighten down the front receiver from the back.

im not talking about the top metal receiver from the bottom plastic portion. I am referring to the front outter barrel assembly from the upper receiver.

i can see a screw but can't access it because of the outter barrel. There is also some sort of push pin on the upper receiver but not sure what it's for.

any ideas?