You could try freezing the barrel in your chest freezer. Then after it is good and frozen(day or so) take it out and apply an immediate heat source. I do not mean a heat torch, just what ever you have that will heat it up fast. And apply constant pressure while it is heating. You might want to take the scope and stock off the rifle. Just be careful not to get it too hot. If that doesn't work then take it to a professional. Tom.
Was it a stainless brake on a stainless barrel? If that's the case the threads might have galled and cold welded themselves.If that was the case I'd probably use some type of anti-seize compound on it in the future.
Tough to understand why a good 'smith couldn't get it off???
did he say why?
If you turned it on, then you should be able to turn it off. The thread on choke tubes and braakes do get stuck but never saw one tha didn't come off with a steel rod into the brake holes and a hard hit with a 2 foot hunk of 2x4.
You must oil the threads and certainly anti-sieze grease (sometimes called brake grease) will work..if you get it off.
some delicate work with a dremel cutter bit down almost to the threads and then use a nut-cracker to break the threaded nut/furrele part of the brake will work to relieve the threads. Chase out the threads and get a new brake.