If there is no evidence of any other mechanical or chemical fixing,then attempt to undo the brake by normal means.This ensures that no exraordinary forces are applied to the barrel.The brake will then do 1 of 3 things.
1)undo normally
2) begin to break up(depending on how far along you cut through to the threads)and fall off.
3) Not budge
If 3, check again for concealed fixings as some brakes are fitted without the pins being visible to the naked eye,remove these if necessary,and try undoing it again.If 3 again,and you are absolutely sure there are no other mechanical fixings,suspect chemical fixing and apply some heat to melt the bonding whilst undoing the brake.If 3 again, use a tapered punch large enough the expand the brake or wedges driven in longitudinally (sorry,lengthwise) to the same effect.This will ensure that no lateral bending forces are applied to the thread/barrel.Try unscrewing the brake again.Progressively increase wedging action until you can remove it or the brake disintergrates.
If 3 again suspect silver soldering and heat to a much higher temperature to melt solder and repeat unscrewing procedure(Try this before the wedging stage!)
If any of the above don't move it then it has been puddle welded or some other technique in that case,cut off as much of the brake as possible without touching the barrel proper,and machine what is left down to 0.5" in a lathe using a dog and rolling centres and centre support bearing,and cut a new 0.5"X28 thread!