I would grab the arm holding the gun, break his arm in half from the elbow, heat butt the perp, flip him around and then snap his neck.
Hey, if it works for Ahnold...
I took judo classes while I was in army electronics school down in Huntsville, Alabama, from this Japanese master who had a studio in town. Maybe I'd grab his arm and try a hip throw. Or maybe I'd just try a good hard chop at his wrist. If it failed, I'd get killed. But it would still be smarter than daring the guy to shoot you so he can prove his manhood.