We are also assuming he was firing 5.56 ammo. He could have had the weapon (chambered in 5.56) loaded with .225 rounds. I did read that the weapon was 5.56.
A key to this case will depend on recovery of all fired rounds. Ballistics should reveal whether they came from the perps gun or possibly some other gun. So far, I haven't heard any mention of this.
I can't believe they abandoned the scene the night following the attack. They should have had a whole team of experts combing the premises throughout the night with metal detectors searching for those rounds. Who leaves a crime scene vacant like that?