Re: Removing Bushmaster free-float tube
They are just on there tight and I have seen a couple of instances where loc-tite was used as well. I would just get a strap wrench from Auto-Zone or Lowes - the kind normally used to remove oil filters - and use it. I have done it in the past on the very setup you are working on with success.
-edit-
Just noticed that you're in Norway, so whatever hardware/tool store you have should have them. The rubber strap won't mar your aluminum forearm either. Good luck.