Wash. Rev. Code § 9.41.122
Residents of Washington may purchase rifles and shotguns in a state other than Washington: PROVIDED, That such residents conform to the applicable provisions of the federal Gun Control Act of 1968, Title IV, Pub. L. 90-351 as administered by the United States secretary of the treasury: AND PROVIDED FURTHER, That such residents are eligible to purchase or possess such weapons in Washington and in the state in which such purchase is made: AND PROVIDED FURTHER, That when any part of the transaction takes place in Washington, including, but not limited to, internet sales, such residents are subject to the procedures and background checks required by this chapter.
RCW 9.41.122
Finding- 2015 c 1 (Initiative Measure No. 594): See note following RCW 9.41.010.