Timely video:
Ken Hackathorn goes into extensive detail about what happens in mass shootings, especially in churches, and discusses core concepts behind stopping an active shooter.
After the Paris Disco attack we set up our own "Disco" with 20 No Shoots and 3 Shoots. The were intermingled together...close...like they were in Paris. We thought a reasonable time to bring the Shooters to heel was 5 seconds before they would start shooting at You. The plan was to simply get 1 shot on each target. We started 15 yards away in a position that would not give you a shot at Any Hostile. When the beeper went you had to hit the 3 targets within the 5 second range. You could move Anywhere when the beeper went.
Over the period of a month....not one person was successful. That is to say nobody did it in time without taking out innocents.