Help removing the "end cap" from my AA TDR

Hi guys, couldn't see a modifications category so thought I would post this here.

I have recently purchased an Air Arms Tdr, loving it but the one I purchased is the shrouded barrel version without the suppressor and it's a pretty noisy sucker. I have a male to male AA thread adaptor and a suppressor but can't figure out how to remove the end cap so I can thread it on.

Any ideas?
 
I don't have this gun but it might have a thread locking compound on it. This means that you could try GENTLY heating it. Use a soldering torch and put the flame onto the barrel for just a second and move it off. Repeat this a few times and try the screw. Take your time doing this. You don't want this to end up smoking and glowing. Use an old towel or cloth for better traction.
Rob.