Have you ever shot a gun? It's not like in the movies where you point, shoot and magically hit your target with immediate fatal consequences. And the farther the target the waaaaaay harder it is to hit said target.
Guy walks into a school or a workplace, stands at the door to an office and turns everyone in the room into hamburger without getting anywhere "near" any of his targets. If he's got clips and mags and a semi-auto he doesn't need to be terribly accurate; he can just keep pulling the trigger. This is decidedly different from the scenario where he must go to each victim individually, come into close physical proximity and plunge a knife into his chest or throat, one at a time.
I'm glad you brought up self defense with knife scenarios. I wonder if in a "mass shooting" scenario if there was someone else there with a gun to defend if the outcomes would be different. Hmmmmmmm......
It's interesting that the pro-gun crowd approaches this problem from the "fight fire with fire" angle whereas others are trying to prevent the tragedies in the first place.
But the facts don't actually support this position nor does correlation (which in the US isn't there as noted in info I've posted) mean causation.
Not going through 8 or more pages of thread to find this but on the face of it I refuse to believe that, if compared, a hypothetical mass shooting carried out with a musket or flintlock would be as devastating and gory as one carried out with an AR-15.