How do I use raycast to simulate gun fire?

try this

put this code on your gun this will fire along the Z axis, blue arrow in the editor from the guns transform position

var damageDone:float = 1;

function FireGun(){

var hit:RaycastHit;
var fwd = transform.TransformDirection (Vector3.forward);
  if (Physics.Raycast (transform.position, fwd, hit , Mathf.Infinity)) {
    hit.collider.gameObject.SendMessage("BeenShot",damageDone,SendMessageOptions.DontRequireReceiver);
  }

}

Put this on your zombies (the game object with the collider)

function BeenShot(Damage:float){
//code for been shot
}

edit: added SendMessageOptions.DontRequireReceiver to send message so you dont get an error when you shoot objects other than the targets head