As any of the officers who shot at Amadou Diallo will tell you, if you're behind cover, you have an opportunity to wait, see what happens, and respond. If you're out in the open, and you feel you're about to come under lethal atack, then you shoot NOW before the attack can start (or re-start).
As any of them will also tell you, expect to be prosecuted. Comes with the hand you've been dealt.
This attacker may be going for his gun, under the seat. Or another weapon. Or just wants to use his car (a lethal weapon) to run you down.
And you're sitting there, unable to run away or take cover, with a two-shot gun. And you have a duty to protect your wife.
This moment may be the only opportunity to end the attack, before he jumps in the car, closes the door (there goes your shot) and runs you down.
So shoot. Shoot him in the back. Because you believed if you didn't, he'd have killed you in the next instant.
And then go stand trial, and pay all those costs. But, tell you what--I wouldn't convict you.
Stay safe.
(BTW: Shoot and miss is what he actually did, and BG ran away? In that case, his solution was better than mine!)