Jinx that is not leather. use an old belt or cut a pice out of an old leather glove. also get some "Kroil" and put it on the barrel just above the nut. soak it down and let it run down inside the nut and sit for about 24 hours and it will help also. be sure and stand gun up and put old rag around action to keep oil off stock and anything you don't want it on. I have found if it ain't supposed to be there that is the first place that get oil on it.
Then put barrel in blocks and wrench on nut grab action with gloved hand and hit the wrench with hammer like it was the most evil mother-in-law (pretend it is my ex inlaws) you can imagine and her hand is on the wrench. Just remember righty tighty...lefty loosy.

Gary