Nipples were always designed to be worn out and replaced, I highly doubt the ones that are on it are original to the gun, but they may very well be old. Shooting a gun with old, worn out nipples is not a good idea. They need to be removed and the threads in the breech plugs need to be examined. Many times they are worn/corroded and might need to be freshened out to a slightly larger size.
There is a company called Track of the Wolf that specializes in muzzleloading supplies. They make oversized 1/4-28 (if that is the original thread size in your gun) nipples in increments of .005. You might need to get one (or more) of their oversized refreshing taps to get a good fit for a new nipple.
I can not stress enough to not shoot antique worn out nipples. If you really want to keep them, remove them and replace with new ones when you want to shoot the gun. When you hang the gun up for display put the old ones back in.
