For the record, a .30-06 scares me as much or more than an AR-15. A man ready to kill, with a plan, behind a scoped .30-06 is terrifying thought. Asshole in a crowded room of people not willing to fight between mag changes is easy pickings, no doubt. But he could have did the same amount of killing with ANY firearm in those circumstances.
In the end, I'll to defer to you on this but please to be explaining. In particular, the "any firearm" statement. Yes, it was obvious that no one in that club was prepared and/or willing to fight back. But they were trying to get the hell out of there. Put yourself in the coward's (Shooters) shoes, with the stipulation of you knowing what you know given your training and experience.
Your plan is to go to this club, where several hundred people are. You want to take out as many as possible, as quickly as possible. You know it will be utter chaos once you start shooting with people running for the exits. What you have available is whatever weapons you can purchase at the local gun shop. You can choose one firearm and enough ammo to reload as often as you can.
I know you can see where I'm going with this but, what do you pick?