Can anyone tell me how to change the amount of damage done to a player based on what limb the projectile strikes?

For example:
Is it possible to make it so that if a ghoul or human head takes over thirty or so damage, it's an instant kill? (I say thirty or so because I don't want random damage from a grenade explosion instant killing them).

Also, is it possible to make certain armors more resistant to certain types of ammunition?