Are NFA items such as a SBR that also fall under the NJ state assault weapons ban required to be compliant with the ban?
There are no NJ-legal SBR's - NJ makes all SBR/SBS illegal in 2C:39-3b, which says "Any person who knowingly has in his possession any sawed-off shotgun is guilty of a crime of the third degree".
So, what does NJ consider a "sawed-off shotgun"? According to 2C:39-1o, it's any shotgun having a barrel or barrels of less than 18",
or a rifle having a barrel or barrels of less than 16 inches, or any firearm made from a rifle or shotgun with an overall length of under 26"
So yes, your legally-registered M4 SBR is illegal in NJ, because it's a sawed-off shotgun. The only exception to this is found in 2C:39-3g - which is member of military or LE on or going to/from duty, and possession is duly authorized.
It's interesting to note, that possession of machine guns is permitted in NJ, if one obtains the proper license (and this one's even more mythical than the NJ permit to carry a handgun). However, no such license exists for an SBR. So, one could theoretically walk around with an M60, but would go to jail for a Form 1'd cut-down 10/22.
