Who gave you this piece of false information? To be NJ legal it just cant have a flash hider or bayo lug.
Incorrect...
The AR-15 and CAR-15 are both "named" weapons on the "assault firearms" list (NJS 2C:39-1w). Therefore, if the receiver is stamped either 'AR-15' or 'CAR-15', it's an illegal "assault firearm", REGARDLESS of configuration.
Now, if it's some other model, it can not fall under the "substantially identical" clause of the same statute above, which has been defined by the AG to mean any semi-automatic rifle with a detachable mag., and at least two of the following: pistol grip, flash suppressor, bayonet lug, collapsible stock.
So, for example, a Bushmaster XM-15, with an A2 stock, and no flash suppressor (muzzle brakes are OK, as long as the ATF agrees that it's a muzzle brake and not a flash suppressor), would be legal.
No one, other than law enforcement or FFL's (or someone who possessed one prior to May, 1990, and jumped through the hoops to register it) can "legally" possess an actual "Colt AR-15" in NJ. (Note that the Colt AR-15 and the Colt Match Target are different rifles - the latter being NJ-legal)