Okay - so this just became law today - now known as 2C:39-3n:
Anyone who may have built a firearm from an 80% receiver elsewhere in this country, and then transported the completed firearm back into NJ - you are now in possession of contraband, and subject to a 3-5 year prison sentence.

I'm not sure if this law can be construed to apply to, say, an antique muzzleloader - which is still a firearm by NJ standards, and was originally manufactured from a firearm frame/receiver without a serial number registered with a federally licensed manufacturer...  because FFLs didn't exist until 1938.  Firearms were not required to have serial numbers until 1968.