What I am GUESSING has probably happened here is that the thread has been stripped by someone over tightening the nut when trying to undo it, not realising that it is a LH thread.

So, if you can get a wedge or claw hammer or something(?) in behind the bracket and apply a force outwards on the bracket away from the barrel as you turn the nut clockwise, it MAY (not definite) start to take up on what thread is still left and start to undo.

Otherwise it may be time to think about using a grinder or similar

Hopefully someone can suggest a more professional method.
_________________
[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...