Hard-Headed is a player perk that reduces player damage from headshots.
Like the description suggests, Hard-Headed reduces headshot damage. It also effectively negates the damage bonus from firearms modded with Hollow Point rounds.
However, Hard-Headed will not protect the user from non-headshots, thus weapons that can kill in a single body shot like the Barrett M82 and L115A3 can still kill in one hit. It is worth noting however, that a one shot kill may not be guaranteed at long range, due to damage dropoff. High Caliber rounds negate the dropoff penalty.