If you know where an opponent is or going to be, throw a flash or concussion, hopefully they will be stunned, so you could somewhat "take your time" and aim up abit.
The AN-94 has a tendency to get heads if aimed at the chest from mid-long range. Shoot and let the recoil work it's way up. (grip helps with this).
If your really desperate (and prepared for some hate mail), grab an 870 w/ laser sight/long barrel on a small map like Hijacked. Keep your cross hairs in the center of the screen, when an enemy appears, simply aim in the general chest/neck area. More than half the time, it'll be a headshot.