Helmet armor and just higher damage to head is punishing good aim of player. Headshot isn't rewarding. Situation like, when I shoot a player 30 meters away from me with Scar-H in the head and his helmet saves him is stupid and one of the worst feeling in game.
I suggest insta dead on headshot depending on the caliber of the gun. For exaple, 9mm weapons 0-75meters, 7,62x51mm 0-300meters ... etc. Helmet will still be able to save you from shot further away. Sniper rifles should stayed the same.