Long arms are not allowed to be carried loaded in a vehicle. I know this was gone over many, many months ago, but I doubt I'll be able to find it with our crappy search engine.
I'll see what I can find for citing source and edit this.
Edit: Here's the law, it's a misdemeanor: RCW 77.15.460
Oh, and it wasn't as far back as I thought, only a month ago *chuckle*: http://opencarry.mywowbb.com/view_topic.php?id=13388