Unless there happens to be a friendly policeman on the scene when the shooting starts you have 10-20 minutes if history is a lesson before the calvary arrives.
If an active shooter is dressed up like the last few, it should be fairly obvious WHO the BG is. I am going to engage anyway, that is not internet commando, that is old cop. Don't believe me? Don't care.
My rifle and pistol are tools, I am the weapon.