A very good question, @Scott361, and I have wrestled with it but not come to any satisfactory conclusion.
In any violent situation everyone present is making split-second decisions, armed or unarmed, perp or (potential) victim, brave or cowardly—everyone is acting faster than the speed of rational thought. Run? Hide? Drop and cover? Make a deal with God? Scream for help? Engage the shooter? I this chaotic soup of terror and adrenaline, how is anyone supposed to differentiate between a bad guy with a gun and a good guy with a gun?
As a legally/responsibly armed civilian, the moment my weapon comes out is the moment I become another shooter in the mix, and only I know what my intentions are. To anyone else, I’m just another guy with a gun in a place where nobody is expecting gunfire. What can I do to set myself apart from the bad actor(s)?
My halo isn’t visible to ordinary folks, so that’s no good.
I don’t usually go out wearing clothing emblazoned with words like “SECURITY” or “ARMED DEFENDER of INNOCENTS” or “GOOD GUY - DON’T SHOOT”, so nothing there.
Since I’m not a policeman it’s illegal for me to identify myself as such, though perhaps under the circumstances that might be forgiven, especially if the outcome is good. If I screw up and hit civilians, they’ll crucify me for it. Maybe shouting something like “Security!” or “Undercover!” and letting everyone interpret it for themselves would work. Of course, doing so might also make the shooter target you specifically. A grey area at best.
Maybe shouting “Help” over and over, as a statement rather than a plea? At least it’s something that wouldn’t be expected from the bad guy.
Shouting “2A” or “2nd Amendment” might work, at least as a code between responsibly armed civilians, but probably wouldn’t do much good with law enforcement.
Shouting anything might not be much good anyway. First, as previously stated, it may just make you a target for the shooter. Second, the danger zone is likely to be very loud and screamy, so you might not be heard no matter what words you use. And under the conditions, who’s listening anyway?
I can hope to be the only responsibly armed civilian in the vicinity, at least for the time it takes to neutralize the threat and re-holster. At least that way I’m not showing a weapon when more armed defenders arrive, law enforcement or otherwise. Hope is a fine thing, but hope is not a plan.
I don’t have the answer. I suppose I can just commit myself to doing the right thing and depend upon my righteousness to protect me. You know, a moral victory at minimum. But that sounds a lot like hope.