Black powder guns, at least ones of antique design (but modern production), are federally ~unregulated already anyways. A 6 year old in North Dakota could order one mailed right now to his house, no background checks, right off the internet -- legally.

There is also the "felon carry" as its called late 19th century black powder percussion pistols, you can also order off the internet, regardless of criminal history and with no scrutiny of the chain of custody.