Hey guys!
http://www.nj.gov/njsp/info/pdf/firearms/njac-title13-ch54.pdf is really helpful in figuring out what is illegal and what isn't.
The parts that applies to this:
13:54-1.2 Definition:
"Antique firearm" means any firearm which is incapable of being fired or discharged, or which does not fire fixed ammunition regardless of date of manufacture, or was manufactured before 1898 for which cartridge ammunition is not commercially available, and is possessed as a curiosity or ornament or for its historical significance or value.
What that translates to:
The state of NJ does not view black powder weapons as firearms to be regulated. I have several lawyers in the family, including one who's tried several cases involving gun owner rights and one directly involving a man with a collection of black powder rifles. His explanation, as well as I can remember it: "The thinking is simple: one, black powder is less powerful, which leads to less inadvertent damage; two, the longer reload time means a patient police officer can wait for you to fire and start to reload to enter safely; and three, it would involve making a lot of people who randomly have a gun as a historical piece, passed down through the family, suddenly criminals."
Basically, black powder weapons are not classified as guns according to NJ law.