What difference does it make whether the gun is stamped AR-15 or CAR-15?
It's one of those pesky legal things... NJ has a ban on "assault firearms". They define an "assault firearm" as belonging to an enumerated list of firearms (statute 2C:39-1w). One of the types on that list is "Colt AR-15 and CAR-15 series". Now, even if you took an actual Colt AR-15, removed the upper receiver, and replaced it with an upper that had no flash suppressor or bayonet lug (ie. a "neutered", post-ban upper), it would no longer be identical to a "Colt AR-15", however, since the lower receiver is still marked "Colt AR-15", it would still be considered illegal in NJ.
Beyond that, the same law states that any firearm manufactured under any designation that is "substantially identical" to any listed assault firearm is also an assault firearm. What the hell does "substantially identical" mean? "Mostly similar"? "Kind of the same"? In an effort to clarify that, the Attorney General released a set of guidelines that effectively said an assault firearm is any semi-automatic rifle that can accept a detachable magazine, and has two of the following features (these have come to be known as the "evil features" list): A folding or collapsible stock, a pistol grip, a bayonet lug, a flash suppressor, or a threaded barrel capable of accepting a flash suppressor, or a grenade launcher. On most AR-15 style rifles, the one allowed "evil feature" is the pistol grip.
So, what this all means, is that a Colt AR-15 is illegal to possess in NJ, while a Bushmaster XM15, with a fixed stock, and target-crown barrel, is legal. Make sense? Of course not - but there you go.
