Okay, this trick may save the day for you. Mix up a 50/50 mixture of acetone and transmission fluid. Plug your nipple with a toothpick and fill your bore with the acetone/tranny fluid mixture. That mix is the best penetrating oil in the world. Let it soak for a few days. Then empty out the mixture and get a small deep socket that will fit on the nipple. It should come out. If that fails, try heating the area around the nipple with a propane torch. You want it just sizzling hot. Check it with a drop of water until the water sizzles. Then apply a piece of dry ice to the shaft of a screwdriver to chill it until it is very cold. Use care in handling the dry ice as it can cause thermal burns to exposed skin. Apply the chilled screwdriver to the nipple to chill it. The thermal shock of the hot barrel and cold nipple should pop it loose. While the barrel is still hot and the nipple is cold, give it another try with the deep socket. If this doesn't loosen it, it is time for a trip to the gunsmith. He will be able to carefully drill out the nipple and use a tap to chase the threads. You are probably looking at a gunsmith's fee in the area of $50 to $75 so if you can get it out yourself, even having to buy some dry ice, it will save you a bit of money.