Consider NYC's law that was passed around 1976. They required the registration of all long guns. Well, as things would have it they took their time and eventually outlawed assault weapons and required the owners to turn their weapons into dealers and get a receipt for them. Just giving, or selling your gun to a relative, or friend outside the city and showing a notarized receipt was not enough. Those who would not comply would have their right to own a weapon in the city voided. That's where they got them. No one would want to have their pistol licenses revoked, or have their other registered rifles and shotguns confiscated as well.
The state is bad enough, but the city is insane. B-B guns have been outlawed since the early part of the 20th century. You're not allowed to receive ammo through the mail. Since last year it's illegal only in the city to possess an open assist pocket knife, or any knife with a blade larger than 3". I wouldn't hold my breath waiting for lawsuits to correct this problem. Once something's registered on paper it pretty much belongs to them and when the rules change they'll do whatever they want. It is what it is.