You are saying he could do as much damage in the same amount of time with weapons that take longer to use and less accurate. I don't think that is possible.
I was saying the same amount of damage or worse could be done with a lesser weapon.
It's entirely possible.
Take a .38 special revolver.
Go buy a flare gun.
Wait near a nearby gas station.
Shoot the driver of the gas truck.
Drive to/into school.
Shot truck with Flare Gun.
Evil does Evil things with a AR-15 or whatever.
We don't know how much planning or time he had put into this.
I can't imagine he woke up last week and said....."I'm going to murder some innocents today"