Jim,
Here’s one of at least two commercially available conversions for black powder to cartridge: http://www.kirstkonverter.com By installing one of these cylinders, one is legally manufacturing their own weapon, so no paperwork is involved. [Under the Federal law] as long as the components are shipped separately, it can be mailed to any adult per federal law. Note however, that some states and localities will have differing laws. – Michael Z. Williamson