This is a text of the law in section 1 of RCW 9.41.190 it states a person may not manufacture, possess, blah, blah, blah an SBR, MG, etc... Below is the text now added to the RCW. So it would appear on it's face, that the law was poorly crafted and an individual may not manufacture an SBR.
(2) It is not unlawful for a person to possess, transport, acquire, or transfer a short-barreled rifle that is legally registered and possessed, transported, acquired, or transferred in accordance with federal law.
http://apps.leg.wa.gov/documents/bil...re/5956.PL.pdf

