Interesting scenario, while it is likely to happen in a "bad" area, could actually happen anywhere. I agree with lima, if my family is with me, I call police and keep them safe, they are my first priority. Secondly it depends upon where, if i am in my town with my normal CC I only have 8 rds, 9mm, a reload and bug. but the first shots must count and add the adrenaline and fear... wow... out side of town i carry higher capacity and larger caliber sometimes, might be more prepared.
I would call 911
make safe, observe what is behind
if need or choose to act, do so from cover, and hit the first target