Helmet zombies are significantly harder to kill because they will obviously ignore headshots, so try to shoot the torso or legs. Their helmets make them look as if they were welders and also come in three variations, bronze, iron,steel,silver and gold.
Although it's doesn't seem to effect bullets in any way as well it's doesn't effect other creatures that use melee attacks.
o Helmet zombie wear biobot helmet (not to be confused with robot helmet)
o Was the first enemy to be appear with critical damage resistance (basically headshots or lucky shots)