Let's not get hung up on the hardware.
Since nobody else was armed, he would have had plenty of opportunity to do this damage even if he had been using a SxS shotgun, a revolver, or even a bolt action rifle.
Let the anti-freedom crowd scream about the weapons, we need to elevate the discussion beyond that point to state that the crazies can't be stopped from doing bad things, and that is why good people need to have the tools to counter them.
It is not the weapon, it is the warrior.