Nipple rusted in place.

[Billnpatti]

A trick that has worked for me is similar to what some have suggested but the difference is that I apply heat with a propane torch to the female threaded part (in this case, the barrel) until it is pretty darned hot but not glowing. I then apply dry ice to the male part (in this case, the nipple) and let the ice work for several seconds and then give the male part a good rap with a hammer to break up the rust in the threads. I repeat this treatment several times before attempting to turn the male threaded part. You need to break up the rust flakes or they will act like small wedges and jam the parts even worse than they were in the first place. Apply a good soaking of Kroil or similar penetrating oil and try working the part back and forth to continue breaking up the rust flakes. Finally reheat and apply the dry ice and attempt to remove the male threaded part from the female threaded part. If it is loose, rock it back and forth as you remove it to break apart the rust flakes keep them from jamming in the threads. If this doesn't get them apart, you will have to take it to a gunsmith. Because of the hardness of the nipple, he will need to have on hand or buy a special drill bit to cut the metal in the nipple.