M9K Damage Values

Can someone explain this, In the configs for each weapon is a line thats says :

SWEP.Primary.Damage		= 15	-- Base damage per bullet

And if you change it, it changes the damage of the bullet that is shot at the person. But there is a problem. I set this to 0 for a pistol in single player. Then i shot the NPC and got a 0 damage on it, like expected but when i shot the NPC head i got a damage value like 100-300+ and sometimes it killed the NPC in like one shot. But only does damage on the head even though the damage configs says :

SWEP.Primary.Damage		= 0	-- Base damage per bullet

How would i change the HeadShot damage value.

When the damage is 0, the FireBullets function will use the npcdamage key for the weapon’s ammo type. Otherwise, the damage could come from a hook that manages head shots.