Gunslinger use a "6 point" socket, that won't round it off like a "12 point". Try 17mm or 18mm, also 11/16, whichever fits tightest.
Don't be afraid to use a socket you have to force on the nut, the tighter it fits the better.
Don't just use brute force in one direction only. Go both ways, back and forth, while you work on breaking it loose. Going one way only, you may twist the stud off if the nut is really jammed.
Heating the nut is a great idea.
The hobby shop may have a tool called a "nut splitter". If so, its an alternative if you can't get it to unscrew off.
Too bad you're not close by, I could have that thing off in a few minutes no matter what (I'm an artist with the "hot wrench" if the air wrench doesn't get it

)
culero