RayCast Problem : 2d

I am making space shooter kind of scene. But in that i am shooting in the direction of mouse position.

All this works fine. i am having Enemy script like this:

 function Start () {
     SetPositionandSpeed();
 }
 
 function Update () {
     var amttomove : float = currentSpeed * Time.deltaTime;
     transform.Translate(Vector3.down * amttomove);
     
     if(transform.position.y <= -6.00f)
     {
         SetPositionandSpeed();
     }
 }
 
 function SetPositionandSpeed()
 {
     currentSpeed = Random.Range(minSpeed , maxSpeed);
     x = Random.Range(-4.00f , 4.00f);
     z = -6.72f;
     y = 8.0f;
     transform.position = new Vector3(x , y , z);
 }

On mousebutton down I am Generating Bullet.

And my shooting script is this :

 function Update ()
 {
     var bullet : GameObject = GameObject.Find("Bullet");    
         var hit : RaycastHit ;
           var ray : Ray  = Camera .main .ScreenPointToRay (Input.mousePosition );
           if(Physics.Raycast (ray, hit)){     
              target = (hit.point - bullet.transform.position).normalized;
           }
             if(target != null){
             var moveAmount : float = 6 * Time.deltaTime;            
             transform.Translate(target * moveAmount);
          } 
          
           if(transform.position.y >= 7.0f)
           {
               Destroy(gameObject);
           } 
 }

But sometimes when my mouse position comes over the enemy , my bullet changes direction slightly.. So what do i need to do? May be Problem with RayCast..

Please Help me to solve out my problem..

Thanks for your support and help in advance..