not sure what the laws are now. but when I was a kid blackpowder guns (non-cartridge) wher not even classified as firearms, just weapons. so they had the same type of regulation as a sword or and air-riffle.
I imagine the laws are still simular, since you can still order black powder guns through the mail, without going through a dealer or a background check.
in junior high metal shop I turned a .75 cal cannon out of brass. shot it fairly regularly until it came up missing some time around my senior year of highschool. I also built a .36 derringer as a craft project in highschool. amazing how much things have changed in the last 20 years.