First find someone with a set of thread guages and find out whether or not this may be a standard thread size. If it is y ou can now modify a new commercial nipple and be on with your shooting. (Look up a local nuts and bolts supplier - they will have all the guages needed. Then go over to Americanlongrifles.com. The continents foremost in muzzleloading arms are on there and will get you going quickly. One of them very likely will have what you need or offer to make it for you.