Unless you have used an easy out before, I would suggest your local smithy. It is very easy to mess up the threads if you don't have experience with one.
The biggest problem I see with ANY powder gun, is people over tightning the nipple. Be it on a rifle or a pistol, neither of them need the nipple to tight, and should be loosened, and lubed after every use of the weapon.
I see this all the time with rifle shooters, that shoot hammer type side locks. They will clean the nipple, by pushing hot water up and down the barrel without removing the nipple, and think they have a clean weapon. This will work OK if you never have to take the nipple out, but they get a big surprise when they have to replace a worn nipple, and figure out that it is frozen solid.