Plunging frontal, oblique and flanking enfilade fire with a significant beaten zone against an area sized target of 25,000 people. That is what automatic weapons fire is made for. What limits the weapon is the lack of a quick change barrel on an AR-15 when it begins to overheat.
Yep. Had the psycho chosen one M1917 watercooled .30-06 or MG08 Maxim watercooled (or Vickers)....he would have killed thousands. Bigger bullet, cloth belts, ability to eject round with pull of handle, unlimited belts, low reload time, very VERY accurate.