PLG I wouldn't want to put heat to it as that will expand the shank into the nut that is welded to the barrel making it tighter. It's not like your trying to take out a rusted manifold bolt. Because of the frame you can't put heat to the nut.

I would try a good quality hex wrench first. If that fails get it off to Smith&Wesson.

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40 yrs Millwright Machinist