I have only removed one barrel nut, a smooth one. A 3 ft piece of pipe on the end of the pipe wrench made it easy-peasy, only after I had put heat on it, PB Blaster, and got an action wrench mounted in the vice.

I know more about getting metal pipes loose from their fittings. My Dad showed me a trick long ago. Put the fitting (in this case the barrel nut) on its side on a hard surface such as an anvil and give it a couple of good licks with a small sledge or blacksmiths hammer, turn it a quarter turn and wail it again. That seems to break loose any rust etc. that may be keeping the threads locked I guess. I have seen that make daylight and dark difference when working with galvanized steel pipe.

BW