A. I’ve got some bad news. You can’t.
2nd: if you think a potential mass shooter is frivolous nonsense, I don't know what to tell you. I personally think it's better call *before* the shooting starts.
But imagine if all 100 people in the grocery store immediately dialed 911 the moment some open-carry dickneck came hoving into view.
There would come a point where it's simply easier to get rid of the guns.
Not in 2019 it's not.