Thing is, no matter what scenario we run through our minds, it most likely won't go as we envision it.
That my friend is a thing called "My gunfight" and they never happen as you imagine.
Use sound tactics, be proficient with your weapon, don't panic.
The police response to these things takes 5-10 minutes usually. If someone engages the BG you can and will stop them from slaughtering more. Look at what happened at Trolly Square.