Why Does This Give a Reference Error

I am trying to create a class to have multiple kinds of guns but when i run this code and shoot it give my the error NullReferenceException: Object reference not set to an instance of an object FP_Shooting.Update () (at Assets/Scripts/FP_Shooting.js:139) please help. (the commented function is just a back up)

 #pragma strict
 
     var debrisPrefab : GameObject;
     var bullet_prefab : GameObject;
 
     var cam : Camera = Camera.main;
 
     var bulletImpulse : float = 100;
     var slot : float = 1;
     var cooldownRemaining : float = 0;
     var cool;
     var ran;
     var dam;
     var mo;
     var gunMode = Input.GetButtonDown;
 
 
 
 //defines guns
 
 var auto;
 
 public class Guns {
     public var debrisPrefab : GameObject;
     public var bullet_prefab : GameObject;
 
     public var cam : Camera = Camera.main;
 
     public var bulletImpulse : float = 100;
     public var slot : float = 1;
     public var cooldownRemaining : float = 0;
     public var cool :float ;
     public var ran : int;
     public var dam :int;
     public var mo : String;
     public var Shoot = function(){
 
         if ( cooldownRemaining <= 0){
             cooldownRemaining = this.cool;
             var ray : Ray = new Ray ( Camera.main.transform.position, Camera.main.transform.forward);
             var hitInfo : RaycastHit;
             if( Physics.Raycast(ray, hitInfo, this.ran)) {
                 var hitPoint : Vector3 = hitInfo.point;
                 var go = hitInfo.collider.gameObject;
             
             
                  var h : HasHealth;
                  h = go.GetComponent(HasHealth);
 
             
                 if(h != null){
                     h.TakeDamage(this.dam);
                 }
             
                 
                 }
             
                 if ( debrisPrefab != null && hitPoint != Vector3(0,0,0)){
                     MonoBehaviour.Instantiate( debrisPrefab, hitPoint, Quaternion.identity);
                 }
             }
 
         }; 
     
     
     public function Guns (cooldown : float, range : float, damage : float, mode : String){
         cool = cooldown;
         ran = range;
         dam = damage;
         mo = mode;
         Shoot();
     
     }
     public function Guns(){
         cool = 1;
         ran = 1;
         dam = 1;
         mo = "auto";
         Shoot();
     }
 }
 
 public var rifle : Guns = new Guns ( 0.1, 100, 10, "Auto");
 var currentGun = rifle;
 /* public function Shoot(){
 
     if ( cooldownRemaining <= 0){
         cooldownRemaining = cooldown;
         var ray : Ray = new Ray ( Camera.main.transform.position, Camera.main.transform.forward);
         var hitInfo : RaycastHit;
         if( Physics.Raycast(ray, hitInfo, range)) {
             var hitPoint : Vector3 = hitInfo.point;
             var go = hitInfo.collider.gameObject;
             
             
              var h : HasHealth;
              h = go.GetComponent(HasHealth);
 
             
             if(h != null){
                 h.TakeDamage(50);
             }
             
                 
             }
             
             if ( debrisPrefab != null && hitPoint != Vector3(0,0,0)){
             Instantiate( debrisPrefab, hitPoint, Quaternion.identity);
             }
         }
     } */
 
 
 
 function Start () {
     
 }
 
 function Update () {
 //define gun slots
 if (Input.GetButtonDown("Slot 2")){
     slot = 2;
     Debug.Log (slot);
     currentGun = rifle;
 }
 
 
 //Auto check
 
     if(currentGun.mo == "Semi"){
         gunMode = Input.GetButtonDown;
     }
     if(currentGun.mo == "Auto"){
         gunMode = Input.GetButton;
     }
 //guns and grenades fire
 
     if (gunMode ("Fire1")){
         rifle.Shoot();
     }
 
     cooldownRemaining -= Time.deltaTime;
     
     if (Input.GetButtonDown("Grenade")) {
         var theBullet : GameObject;
         theBullet = Instantiate (bullet_prefab, cam.transform.position + cam.transform.forward, cam.transform.rotation);
         theBullet.rigidbody.AddForce( cam.transform.forward * bulletImpulse, ForceMode.Impulse);
     }
     
 }