A head shot is a one shot kill, but they are hard to pull off on a consistent basis. Aim for the chest or lower abdomen. The kick will raise your shots up through the hit boxes.
You can also switch to single shot for long range shots.
On an average day in Jalalabad, I kill more than I die. On a bad day I can't break even. On a good day I have been accused of hacking. there's nothing like dropping around a corner and wiping out an entire squad. But, then I usually play infantry only and am used to ground-pounding.