Red herring. The fact is that it does happen and when a shooter is reloading he's vulnerable.
Just because you would like to pretend all victims everywhere are utterly helpless that's not the case.
Well you're pretending the opposite. That anyone being shot at by a killer need only "man up" and jump the guy when he's reloading. Furthermore, you're making the case that going on the offensive against the shooter is not just possible, but the
preferred response as directed by professionals.
Now, since nobody else is providing much more than speculation, I'll go with more personal experience.
I work in radio, so I've had some experience with crazed "fans" and other associated threats. Worked at one station that was shot at. Worked at another where a bomb threat was called in. Had people threaten me, my co-workers, and the station(s) with violence. Had to deal with stalkers. Had crazy people in the lobby - fortunately not armed , but certainly disturbing. Thanks to incidents such as these, the place I work has a protocol. If a threatening or violent person shows up in the lobby, the receptionist hits the alarm and gets under her desk. When the alarm goes off, everyone is expected to rush the crazy person and tackle him.
Actually that's not true. Our company's security consultants told us in no uncertain terms that the safest thing to do is to find the nearest lockable office, get inside, lock the door, get down, and stay down until the all clear is given. The alarm sends a signal to the police, of course.
The reasons for these procedures are twofold. First off, it is a liability to the company if we don't have some system in place to protect employees in the event of a threat. The second is that the professionals we hired to develop the procedures and beef up building security made it clear that confronting an attacker is the absolute last thing you want to do...especially if he is armed and you are not.
So when the alarm goes off, I don't put on my hero hat and wander out towards the lobby looking for an opening. I lock my doors, and get behind my very thick soundproof walls.