Go test it out. Start a campaign, or an onslaught map. Pick up a rifle and start unloading some sniper shots into the faces of grunts. VERY OFTEN, the shot will go directly through the face of a stationary grunt, or one that is running straight at you and they will not take any damage or die.
I'm not sure if it's just the hitboxes on grunts, or if it's a hit scan problem. I think it is hit scan and a grunt box problem, since I frequently have headshots not kill/damage people when they should.