Consider this: I am, or have pumped gas, and am standing beside my Jeep. I am approached by a would be robber/car jacker. He keeps his gun behind his thigh out of my view, then suddenly thrusts his gun in my face. I have not signaled that I am armed, then push aside his gun while drawing and firing one shot, or follow up shots as required.
Push aside the gun, draw your own and take a shot? Good luck with that Dirty Harry. Maybe you should have kept your head on a swivel and avoided the entire scenario in the first place.
This whole thread is rather entertaining.