First off, it is illegal to have a loaded rifle in a vehicle so you cannot transport it loaded. And in State v. Spencer he was charged under RCW 9.41.270 for carrying a visibly loaded AK-47 as he "walked fast with his head down". I have seen nothing that outlaws open carry of a rifle, however case law could be against you.
As a side note, I don't see anything against a hostered AR-15 pistol if you are trying to push boundries. It should be treated as a pistol while being almost guarenteed to get you noticed by the local LEO.