It would appear that Washington State doesn't recognize that option. Here is the Law:
"RCW 9.41.190
Unlawful firearms -- Exceptions.
(1) It is unlawful for any person to manufacture, own, buy, sell, loan, furnish, transport, or have in possession or under control, any machine gun, short-barreled shotgun, or short-barreled rifle; or any part designed and intended solely and exclusively for use in a machine gun, short-barreled shotgun, or short-barreled rifle, or in converting a weapon into a machine gun, short-barreled shotgun, or short-barreled rifle; or to assemble or repair any machine gun, short-barreled shotgun, or short-barreled rifle.
(2) This section shall not apply to:
(a) Any peace officer in the discharge of official duty or traveling to or from official duty, or to any officer or member of the armed forces of the United States or the state of Washington in the discharge of official duty or traveling to or from official duty; or
(b) A person, including an employee of such person if the employee has undergone fingerprinting and a background check, who or which is exempt from or licensed under federal law, and engaged in the production, manufacture, repair, or testing of machine guns, short-barreled shotguns, or short-barreled rifles:
(i) To be used or purchased by the armed forces of the United States;
(ii) To be used or purchased by federal, state, county, or municipal law enforcement agencies; or
(iii) For exportation in compliance with all applicable federal laws and regulations.
(3) It shall be an affirmative defense to a prosecution brought under this section that the machine gun, short-barreled shotgun, or short-barreled rifle was acquired prior to July 1, 1994, and is possessed in compliance with federal law.
(4) Any person violating this section is guilty of a class C felony."Unless you are selling to the military, police, or exporting, you can't even have one in your possession. Since there were none registered (no CLEO signoff) prior to the laws effect in '94 that exemption is moot as well.
Lets face it, they don't want anyone to have a Class III weapon in Washington, Legally that is. Wonder how many there really are in private hands though.