(3) "Assault weapon attachment" means any device capable
of being attached to a firearm that is specifically designed
for making or converting a firearm into any of the firearms
listed in paragraph (1) of this subsection (a).
How is a complete lower an attachment?
You would be still required to register your "assault weapon attachment" parts, but not firearms that are not semi-automatic.
Remember, this information is coming from Washington Gun Law who is a lawyer.