Automatic Shooting IF enemy is in range.. like DEAD trigger

sorry for English..........!!!!!! please Give me script in..the gun automatically start shooting if Player aims.... I am using RAYCAST shooting....

 #pragma strict
 
 var theDammage = 100;
 
 function Update () {
 
 var hit : RaycastHit;
 var ray : Ray = Camera.main.ScreenPointToRay(Vector3(Screen.width*0.5, Screen.height*0.5, 0));
 
 if (Input.GetMouseButtonDown(0))
 {
 if (Physics.Raycast (ray, hit, 100))
 {
  hit.transform.SendMessage("ApplyDammage", theDammage, SendMessageOptions.DontRequireReceiver);
  }
 }
 }