Hi Nathan, yes sir, this is WA legal. Read section .iv - "A semiautomatic, center fire rifle that has the capacity to accept a detachable magazine and has one or more of the following:" etc, etc, --- Note it says center fire rifle, it does not say rim fire.
budsgunshop.com/feeds/state_reg/washington_restrictions.pdf ,
James' quote below was taken out of context, you need to read the whole thing. It further states "To purchase semi-automatic rifles as defined above, a person must be a resident of the state and minimum 21 years of age." So, a rimfire rifle can be defined as an assault weapon, but if you are 21 or older, you can purchase it.
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If you are still unsure, you can always check with a local FFL.
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Stay safe & have a great day!

Hi Nathan and welcome. This rifle as configured is not legal in Washington. There are no exceptions for military or police members under the age of 21, and .22 caliber
semi-automatic rimfire or centerfire rifles are included within the scope of the regulation. Cheers and stay safe! Details under Rifles here : budsgunshop.com/feeds/state_reg/washington_restrictions.pdf />