Sharp Shooter Achievement
50 headshot kills performed in the single player campaign.
This can be done with any weapon on human enemies as long as you get the "headshot" bonus points for experience (shows up on top left when you get a kill). While going for your other weapon kills, you should have no problems getting this. The easiest kills for this are probably the unaware enemies but you should have no issues aiming at other enemies while they take cover. Upgrading weapons can help, but not necessary.
Note: You cannot reload checkpoints to gain extra kills towards your stats. These must all be done on unique enemies.