It depends on how it is pinned and welded... If it is a blind pin I have become fond of the split and pry method and I have cut four off my various uppers now and I have a Buddy that I am going to get his off of soon as he lets me at his!
www.quarterbore.net/forums/showthread.php?t=153If the break is pressed on or uses pins that you can see odds are good the barrel is not threaded underneath the break. In that case you need to find a machinist that can chuck your barrel up in a lathe and cut new threads into the barrel. I have an AR-10 barrel of this latter "non-blind pin" type and I was a little less then happy to discover that my old dremel couldn't do this one for me.
Good luck and have fun... a dremel a few six packs of bear and you will either have the break off there or you will just need to buy a new barrel....
Just kidding... It is easy to do unless it is one of the pressed on types!