How many hits for a headshot?

I'm not sure if this is a hit registration issue or what. Using the kord, I put at least 10 shots on someone's head about 10 feet away only to not get a kill, but still be killed by them. This isn't the first time this has happened either. It showed hit markers, but were they actually registering? So how many shots should it actually take to get a head shot? It sure as hell shouldn't take 10+, that's for sure...