You don't say?
If you're serious about it though, that's how all FPS games work. Aiming for the head is the most effective way to kill something, be it man, zombie, or whatever.
I'm pretty comfy with the damage personally. It's those one-hit kills that makes me cringe instead of a lengthy firefight. Besides, it doesn't last that long if you aim correctly.
Sadly if you're not good at aiming with a gun, those splashy explosives are not that great at killing people.