New Jersey has a ban on "assault firearms" and "substantially identical" firearms the state considers to be "assault firearms". No one may own parts of an "assault firearm" if the parts can be readily assembled into an "assault firearm".
"Assault weapon" owned before May 1, 1990 are legal as long as they are registered with the state. Police can own "assault weapons" for on duty purposes. If their agency recommends it they may own personal "assault weapons".
Some states have restrictions, banns, or require owners to register firearms they conceder to be "assault weapons". The definitions of an "assault weapon" varies widely from state to state. These definitions can include whether the gun is fully automatic, the magazine size it can carry, and others.