The Feds require that all stocks be extended before measuring OAL - so federally, this would be a rifle (built on a pistol receiver).
(To measure overall length, do so along "the distance between the extreme ends of the weapon measured along a line parallel to the center line of the bore." On a folding stock weapon you measure with the stock extended, provided the stock is not readily detachable, and the weapon is meant to be fired from the shoulder.) ie, not a buffer tube
WA law doesn't clarify this, nor is there precedence that I know of, but that's the reason for asking. Also, WA doesn't have the whole "constructive possession" statutes that I'm aware of so a collection of parts or the ability to take a buttstock isn't at issue either (see all those Lugers, HiPowers, etc... with shoulder stock tangs).
So the funny part would be, if the stock is closed, it
shouldn't fall prey to "intended to be fired from the shoulder", but could qualify for "or is designed to be held and fired by the use of a single hand".
Or with the buttstock extended, it is a shoulder-fired weapon with a 16" bbl and 26" OAL, so its a rifle...
And
United States v. Thompson-Center Arms Company (1992) resolved the "pistol can be a rifle/shotgun and back" issue, just not the other way around.