A kevlar vest could enable an individual to last longer before either he is killed by someone else or he runs out of targets and turns his weapon on himself. "At once" was very poor phrasing on my part and does sound very bombish. I was thinking more that the event would be over quickly, but in that time many could die before the killer could be stopped.
All the more reason to have armor-piercing bullets. Besides, you don't think a nut who wants to go on a homicidal rampage doesn't mind being shot in the arms, legs, or head?