I say it makes a lot more sense that Martin felt threatened, decked Zimmerman, who crumpled, and Martin got him in the mount and started wailing on him, or at least grabbing onto his head/wrestling/whatever. During this, I think it would be very hard for Martin to have not realized Zimmerman had a gun if Zimmerman was carrying it in the front of his pants, considering Martin would probably be sitting directly on top of it. I imagine Martin realized what it was, what it meant, and started to go for it. That's when it turned into the "turning point" situation that the defense keeps talking about. Martin was probably grabbing onto the barrel of the gun, which is why the slide potentially did not fully actuate. The shot to the lower sternum would be about consistant with this position, as well. Because of how quickly Martin seemed to have succumbed, I'd venture to say he was hit in the aorta. The severe drop in blood pressure would have knocked him out in seconds.