Always remember the 21 foot rule.
I think cops are taught this in their training, not sure, not a cop.
Anyway, inside of 21 feet, an attacker can charge and make contact before the attackee has time to react. If he is already aiming and fixing to fire, it may be too late, but what do you have to lose?
Military training, contact sports, martial arts, etc all help. Any plan of action is better than no plan.
For me, some military training combined with 9 years of intense contact sports helped in my reaction/inaction instincts, but working livestock for 15 years , often alone. is what taught me how to confront a dangerous situation with little hesitation.
My understanding is the “momma bear” instinct some mothers have allow them to punch way above their weight class, and overcome situations where they are heavily outmatched.
Having taken hits and been knocked unconscious in the past he[p to show you that it isn’t that bad, often not even felt in the heat of it.
Don’t run up to the threat, run through and over it. This mindset helps as well, but takes some learning/conditioning.
That said, what would I do if trapped by a shooter? I do not know. I have only been shot at once and it made me mad, not scared.