There is actually plenty of precedent for charging a person who illegally supplies a gun to a person that is legally barred from possessing a firearm as an accessory to murder if that gun is used in a homicide. New York state has a safe firearm storage law, NY Penal Code 265.45 requires firearm owners who either have minors, or adults who are legally barred from using a firearm living with them, to keep firearms securely stored in a locked container or safe. If a parent owns a firearm and one of their children is able to obtain and use that firearm because it wasn't properly stored they are criminally liable. If they knowingly give a minor, especially a mentally unstable minor, a firearm that raises their criminal liability even more. In addition New York City has additional firearm storage laws that require firearms not being carried to be stored in a locked safe, unloaded, and the ammunition for the firearm to be stored in a separate locked container.