I'm not a Washington state resident, but the pistol class of AR currently interests me.
Found this:
(2)(a) A person shall not carry or place a loaded pistol in any vehicle unless the person has a license to carry a concealed pistol and: (i) The pistol is on the licensee's person, (ii) the licensee is within the vehicle at all times that the pistol is there, or (iii) the licensee is away from the vehicle and the pistol is locked within the vehicle and concealed from view from outside the vehicle.
here:
http://app.leg.wa.gov/RCW/default.aspx?cite=9.41.050
I suspect the law is equally clear in my state. It goes to researching the specific statutes yourself in your jurisdiction to understand exactly where the patchwork quilt of laws applies.
At one time my state didn't allow CCW, and the municipality didn't allow open carry at all. Now it's totally reversed - but transportation laws are still restrictive. It should be obvious that someone allowed to conceal carry knows better, but in other situations simply having a firearm within easy reach is prohibited. One reason for that are those who hunt from vehicles by simply poking the weapon out the window and taking a shot from the road - which isn't restricted from being seasonal. It's a matter of year round opportunity, and may not even be of malicious intent if it's a rancher/farmer trying to hit a coyote harrassing his livestock.
Problem being that sometimes the incident is ill timed or a poor decision is made, and that creates the restriction on ALL of us. Like running a stop sign - out on a country road in broad daylight and excellent visibility for miles, some do. In other circumstances, not so much.
Whether you or I can carry a loaded AR15 pistol in the vehicle will likely go to it being "readily accessible" and if it's in some kind of case. So, you have to research your laws and interpret the case law and it's nuances in your jurisdiction to see if it's a problem for you.
And others who've posted they are toting their fold up AR15 pistol in the saddlebag of their motorcycle are making their decision. You may, or may not, be able to - but one thing is for sure, asking on the internet won't be a defense in court. You gotta find out your laws in your state for you, not what some guy like me in MO thinks.
Goes to: the OP bought the pistol in Washington State, his log in block suggests he lives in NY and CT. Are we advising him correctly, and what is exactly the state he lives in?