Here is an alternative: Get a large open end wrench that just fits over the barrel nut. Or, an adjustable it it isn't too loose. Get two pieces of 1/8" brazing rod. Put the wrench over the nut, slip the pieces of brass rod into opposite notches, in such a manner that the wrench catches them. You can tape them to the nut if you wish. The wrench will grab the rods, and apply torque to the nut. This will usually work, unless the nut is very tight.