If there is the least bit of rust on those threads it might not come off.
Try heat. If you are getting to the point that nothing you do works, then the only way to do it is the cut it in a lathe.
First try cutting a ring around the barrel right at the receiver. Sometimes this will give you enough relief that you can unscrew it.
It that doesn't work, you'll either have to part it off in the lathe or saw it off and bore the remnants out of the receiver.
I've got to where I don't spend more than 15 minutes trying to take one off before it goes to the lathe.

If it is expoxied on, you run a the real danger of trashing out the threads in the receiver trying to twist the barrel out. You sure don't want to do that.