Sadly, the shotgun is what is going to be blamed here... although I would find it ironic if it were the handgun he used to shoot himself with. Either way, the shotgun will be the bad guy.
Thank God no innocents were physically harmed.
Somehow the evil assault rifle will find its way to the front of the blame train. Shotgun, what shotgun?!?!?!?! He shot himself with an "AR". That's how our media will report it.