If you're able to, why not buy one here in Wa? If sales tax is the issue, buy from a private seller. If price is the problem, I know of several places that have really affordable ARs..
You can buy a long gun in Oregon from or through a FFL. When you take the rifle back to Washington I believe you are still subject to their tax. Paying it my be a different matter.
If you buy from a Private party in Oregon the transfer will have to go through a Oregon FFL. Where you will pay the transfer and BGC fee's
You live in Snohomish. Go to Epic Firearms in Monroe and talk to Roberto. Tell him what you want, and what you want to spend, and he can find it for you. I know the guy, and i've done a lot of business through him. He's trustworthy, and he knows his ****.