GTA enemies were already pretty smart by the time IV dropped, and I can only assume theyll have gotten smarter in V. It doesnt always pay to go for the head-shot sometimes foes will attempt to keep their cowardly asses in cover. Be proactive. Shooting them in the foot, legs, shoulders, balls, whatever is out of cover, will cause them to stagger. Then you go for the headshot. Or you could always shoot them in the balls some more.