It's an inherently difficult thing to search for. "AR-15" is a registered trademark of Colt, but it's been a while since they made the majority of civilian AR-15 pattern rifles (for that matter they lost the M16 contract to FN years ago, and now aren't the exclusive M4 carbine manufacturer).
You might have better luck substituting "assault rifle" or maybe "assault weapon".
I can remember reading at least one, I think two cases with AR-15 pattern rifles being used outside the home, a few more if you extend this to general civilian versions of assault rifles, like in the LA riots.