M16A2 or M4 modified like almost every model the Fed owns... for three round burst. It has been reported on fox via another FReeper that this “AR15” was so modified. That means it was not an AR15.
This guy engaged in, what is known as, "butter knife warfare". You take your butter knife and walk up, all innocent and non-threatening like, to a soldier or LEO or other armed individual and kill him with it and then you take his sidearm. Then you use his sidearm to go kill someone else that has a machine gun and then you have a machine gun.
That's a narrative they don't want.
The AR-15 cannot be “modified” to do three round bursts, otherwise everyone would be doing it.