How about this Armchair barristers...if you leave a State such as Nj does the registration of a particular item and what it encompasses follow you to another State? yes. If you were to sell a Modern and it was done through a FFL in your new State that Weapon now comes back to them the Dealer you sold it to. This does not include Black Powder Firearms sales under the same circumstances. Black Powder Firearms sold to you in N.J. will not come back to you even if there are no regulation,s in your new State requiring any kind of transfer to someone whom can own it. In N.J. all that is required is a N.J. Foid card at point of Sale. And that sale record is kept at the dealer,s or point of sale. No 4473, required. So any transfers on any N.J. brought black powder weapons in another State that allows the transfer is Legal . Moderns and 4473,s are a different Story. Some States consider Black powder Firearm,s as such and are subjected to the same regulations as Modern,s. If the sale was reversed then yes it would have to go through a NJ FFL dealer.
Footnote: It would be easier to find the Holy Grail than to find someone who builds muzzleloaders from the ground up in N.J.