Everyone's saying the guy used an AR but given the relatively small number of shots and the fact that the bullets went everywhere except where the guy was presumably aiming my bet is he was a Socialist Rifle Club guy with an SKS.

Y'know, this is in this weird spot where it's right between canon and as seen (which, by the way, can be explained even with canonical things). If it was truly as seen, the shots would have gone way wide. If it was truly canon, the Reps would be scrambling for a new candidate now.

The sketch! He indeed was stormtrooper.