firing a shell from a tank turret

hey guys, im trying to have a tank shooting, so a have a tank which has a turret, the end of a turret has an empty object from where i the shell spawns and im using this script:

 public var shellSpawn : Transform;
 public var shell : Transform;
 
 function Start () {
 
 }
 
 function Update () {
 
 if(Input.GetKeyUp("mouse 0"))
 {
     var object = Instantiate(shell, shellSpawn.transform, shellSpawn.rotation);
     
     object.
 }
 
 }

but the problem is that for some reason the shell dosnt allway spawn where its supposed to spawn

here is the code how i rotate my gun:

 public var gunPrefab:Transform;
 private var gun:Transform;
 
 
 function Start () {
 gun = Instantiate(gunPrefab,transform.position,transform.rotation);
 gun.parent = transform;
 
 }
 
 
 
 function Update () {
 
 var temp : Vector3;
 temp.x = Input.GetAxis("Mouse Y") * (-1);
 temp.y = Input.GetAxis("Mouse X") ;
 temp.z = transform.rotation.z;
 
 
 if((temp.x > 0 && gun.localRotation.x > 0.10) || (temp.x < 0 && gun.localRotation.x < (-0.10)))
 {
     temp.x = 0;
 }
 if((temp.y > 0 && gun.localRotation.y > 0.15) || (temp.y < 0 && gun.localRotation.y < (-0.15)))
 {
     temp.y = 0;
 }
 
 
 gun.transform.Rotate(temp);
 }