Except as provided in subsection (b) of this section, a person may not: transport a rapid fire activator into the State; or manufacture, possess, sell, offer to sell, transfer, purchase, or receive a rapid fire activator. This section does not apply to the possession of a rapid fire activator by a person who: possessed the rapid fire activator before October 1, 2018; applied to the federal Bureau of Alcohol, Tobacco, Firearms and Explosives before October 1, 2018, for authorization to possess a rapid fire activator; received authorization to possess a rapid fire activator from the federal Bureau of Alcohol, Tobacco, Firearms and Explosives before October 1, 2019; and is in compliance with all federal requirements for possession of a rapid fire activator. Nothing in this section is intended to prohibit the otherwise lawful possession of a part or combination of parts by: a police officer or other person employed by a law enforcement agency under circumstances in which the officer or other person possesses the part or parts in accordance with authorization given by the law enforcement agency; or a person who has registered a machine gun with the Secretary of State Police under § 4–403 of this title if the part or parts are possessed for use in only, and necessary for the proper function of, a machine gun that is currently registered by the person under § 4–403 of this title.