It is ignorant speculation that his other weapons were "lesser firepower" than the AR. As Stenz pointed out, a shotgun would arguably be more effective in such a space.
ETA: this sort of speculation implies that an AR platform is somehow more deadly than a shotgun or a handgun. This line of thinking leads to banning of specific weapon types.
I like how people want to ban hi cap mags too. People at work were talking about how we should regulate the sale of ammo and hi cap mags. I don't know how I work with these people but I guess they like the game of chance, if you give me less that is the lower the chance I have to defend myself.
"I have a very strict gun control policy: if there's a gun around, I want to be in control of it."
- Clint Eastwood