Bullet does not move forward

i have written code for firing bullet. but it does not move forward. when my bullet is instantiated, It appears infront of my gun. So what can be the problem? pleaze help me,.

Here is my code:

 #pragma strict
 
 var BulletPrefab : GameObject;
 
 
 function Update () {
     
 }
 
 function OnGUI()
 {
     if(GUI.Button(Rect(10,10,90,50),"Fire"))
     {         
          var bullet : GameObject;
          bullet = Instantiate(BulletPrefab, gameObject.Find("Spawn").transform.position , gameObject.Find("Spawn").transform.rotation);
             //bullet.rigidbody.AddForce(bullet.transform.forward * 5000);
             bullet.rigidbody.useGravity = false;
             bullet.rigidbody.AddForce(bullet.transform.forward * 5000 , ForceMode.Impulse);
     }
 }

please solve my problem. Thanks for your help and support in advance.