How to rotate a GameObject around another object according to the mouse cursor position in C#

Hi Unity Heroes. I'm trying to rotate an object based on mouse cursor movement(like bubble gun in bubble shooter game). But I'm Filed to do that with my script.

MY script is :

 mouse_pos = Input.mousePosition;
 Debug.Log(mouse_pos);
 object_pos = Camera.main.WorldToScreenPoint(target.position);
 mouse_pos.x = mouse_pos.x - object_pos.x;
 mouse_pos.y = mouse_pos.y - object_pos.y;
 angle = Mathf.Atan2(mouse_pos.y, mouse_pos.x) * Mathf.Rad2Deg;
 transform.rotation = Quaternion.Euler(0, 0,-angle);

It rotates only its same position it won't move. Please any one help me to solve this. Or point me to any good tutorial ....