If I were ever in a situation where I had to shoot somebody to defend myself, if they didn't go down on the first shot and instead made a kind of "gong" sounding noise, the next shot would be to the head. Even if they didn't go clank when I hit 'em, if they didn't go down from a solid chest hit then a head shot would be coming next. In self defence, you don't shoot to wound. You shoot to incapacitate and stop the attack. If they're still standing, they're not incapacitated.