Well intentioned, but still focused on pre-crime.
The common key element to all this stuff is the Victim Disarmament Zone, whether it's school children, or soldiers.
Pretty much this. The disaffected people who do this are usually trying to get one last hurrah of power, dominance, and control in a life they've felt to be weak and powerless. Just adding in the
possibility of uncertainty that some random intended victim will shoot back ruins it for them. They don't want to fail at what they feel is one last chance at some sort of success to cap off their life with.
Of course there are police and armed guards, however, they can be reduced by the shooter mentally as "known quantities" i.e., the guardpost or uniform which allows for a "shoot me first" protocol which makes the potentially more even conflict there bearable because they as the initiator have the advantage of surprise etc.
Introduce that level of uncertainty everywhere, and while I'm sure not all will abandon their plans for spree shootings, a great deal of them will just decide to kill themselves alone instead.
RANT/
The guy two offices down from me just walked into my office asking if I'd heard about that crazy guy in DC with the AR15 and the shotgun. I informed him there was no AR and that the guy only had an 870. He responded to me that the news said he had the AR and that the shotgun he used had some special short barrel to make it more lethal, but he couldn't understand how he could kill someone with birdshot from four floors up, so he must have had an AR. This coworker bird hunts more than I do and uses some old Ithaca pump with a 24 inch barrel. When I explained the concept of using buckshot. he looked like the hamsters on his brain treadmill were on speed. He didn't believe me when I said the shooter used the shotgun to take the pistols of onsite security. I'm shutting my door for the rest of the day.
/RANT
I'd needle that guy
mercilessly. About how "Riot shotguns should be restricted because of how they can shoot indiscriminately in a spread, and how most people don't know they have a BORE EVEN BIGGER THAN FIFTY CALIBER!" etc.