I'd sink a large eye-screw into a stud in the closet wall, hang the AR using the carrying handle, with the cable threaded through the action, and some kind of ammo carrier attached to the weapon.
SHTF, you unlock and go, loading on the way. The cable lock through the action disables it from little hands, and although easily defeated, having it locked to an anchor point will at least discourage the casual unauthorized taking of it.
I'd have two keys, one offsite in a safe location, the other on my key ring.
p.s. I think the AR is a fine choice for home defense. I'd have a tac light on mine. If only I could have an AR here in Kalifornia....
