Basic rule: Never have a round in the chamber unless you're in an active engagement area and the weapon is in your hands. Rounds in the chamber and the hammer cocked is how LEO's or others get killed. NEVER, I say again, NEVER, carry a firearm that has a concealed hammer with a round in the chamber unless you're out of the car and using it. What happens if your safety notch is a little worn and you go through a big pothole or a really rough road or you forget to apply the safety? You have a uncontrolled discharge. And there is no excuse or reason for an uncontrolled discharge in any firearm, moreso a duty arm.
Carry it with a full mag, and empty pipe, in the shotgun type mount for the ar, some even fit pretty nicely in the shotgun clamp as it is, or with minor modifications. It takes about 1/2 second to pull the chargng handle, and if you think you may not have time to do that you need to reasses your necessity and training in carrying M16/ar rifles in general.
Too many officers just seem to think it's "cool" to have an AR hanging in the window, it's a tool that will kill you or someone else if it is used incorrectly and if those utilizing that tool are not trained fully by competant training people the department is making a mistake in putting them, the AR's, on the street.