CYMA MP5k / Silencer?
 

I am trying to install a silencer and noticed that the adapter didn't turn all the way into the gun.

There is a hex key underneath the front of the gun and none of my hex keys fit to unscrew it.

Is it a special key that I need?

It also took me about 1/2 hour to clear the glue that covered the hex bolt - is that normal for imported guns now?

Glue on muzzles. Especially orange ones is normal had been for a while.
The grub screw is probably a 0.5 mm or 1mm

There's a thread protector on top of the threads, just use a pair of pliers to get it off. If it's a clone of the TM, there should be no grub screw on the thread protectors, the screw's for the entire flash hider assembly.

I couldn't retract it all the way out and the screw is somewhat stripped.

Looks like my adapter will not be sitting properly for the duration of its life :(