If the moron is standing right next to you, pointing at you with his arm extended, fast and hard hit to his arm and get inbetween his gun and him.
i took Krav Maga for like a year or something but the first thing you'd do is to 1. make the gun point away from your face, 2. Grab the back of the gun with the other on the top of the gun (I think)
3. Twist it out of their hands, grabbing it
4. Shoot them in the loving face
Assuming that they miss the first shot and concurrents, that is