I was using the auto shotty last night and got 140 headshots and then I got raped by a dart grenade guy.
It's lots easier if you get to dual wield the lever shotgun, and just aim SLIGHTLY above the head. With dual wielding, your fire rate is much easier, and if you're right up to them, shooting above the head makes ALL bullets go to the brain, ensuring a headshot. BEWARE of the enemies with the auto shotgun because get too close, and you gon' die.