destroy gameobject on collision

i have the bullets for a gun destroying after 5 seconds but i want them to destroy when they collide with something here's the script so far

 #pragma strict
 
 var bullet : GameObject;
 
 function Start () {
 
 }
 
 function Update () {
 
    
    Destroy(gameObject,5);
 
 }
 
 function OnCollisionEnter(collision : Collision) {
 
          Destroy(gameObject);
 
 
 
 
 
 
 }

it destroys after 5 seconds but it does not destroy when it collides with a collider any help on what is wrong with it would be nice