On my CA mp5 w/ tac light i dont know how to remove the flash hider
First check weather or not you are turning it counter-clockwise (you want to).
The F-Word: "It's like the Swiss Army Knife of words"
if its the metal one not the plastic one make sure the grub screw is loose.
Also check and see if there's a small hex screw in the flash hider. If you forceily remove the hider with this thing still in, you'll end up stripping the thread right off.