Is the gun in question here a muzzle or breech loader. original post just said hammer gun. Many breech loading hammer guns have a part which threads into the breech to secure the firing which might be referred to as a nipple. I was under the impression this was the case on the gun in question. If so the nipple would not be exposed to the actual thrust of firing, except possibly in the case of a ruptured primer.
Are the threads mostly worn in the breech or on the nipple itself? If you still have some decent threads left in the breech someone with a small lathe should be able to make a new nipple rather easily.
