The encouraging news is that since Va Tech cops have been training to take on and take down the active shooter using the first, or first few, cops to arrive on the scene. There has been a fair amount of publicity about this in the last two years.
On top of that, both police formal training and police journals are covering the issue of how to deal with the legally-armed civillian shooter who is also on scene and actively engaging the BG(s). Boiled down to the simplest terms, they are being trained to look at behavior and technique - the one standing out ion the open shooting towards to crowd running away is probably the BG while the person hunched down behind cover trying to get a clean shot is most likely the legally-armed civillian trying to protect innocent third parties. My guess is they will be mightily confused if both sides are hunkered down exchanging aimed slow fire.
Alo I can say is no matter what I am
not getting one of these.
http://www.dsmsafety.com/home.html
stay safe.