Object rotate to other object direction ?

Hi
My problem that object not follow other object direction. It keeps shaking even if target facing object "gun" !!. if angle over 20 degree , object "gun" follows the main body "vehicle", but it doesn't. How to solve this ?

Code (CSharp):

1.     public float z_angle,min_z,max_z;
2.     public float y_angle,min_y,max_y;
3.     public float x_angle,min_x,max_x;
4.     public float speed;
5.     public Transform target;
6.     public Transform main_part;
7.  
8.     private Quaternion rots;
9.     public float target_compass;
10.  
11.     // Update is called once per frame
12.     void Update () {
13.  
14.         target_compass = Vector3.Angle(target.transform.position - transform.position, transform.forward);
15.  
16.         Vector3 charAngle = gameObject.transform.eulerAngles;
17.         charAngle.z = (charAngle.z > 180) ? charAngle.z-360 : charAngle.z;
18.         charAngle.z = Mathf.Clamp(charAngle.z, -min_z, max_z);
19.         z_angle = charAngle.z;
20.         gameObject.transform.rotation = Quaternion.Euler(charAngle);
21.  
22.         Vector3 charAnglex = gameObject.transform.eulerAngles;
23.         charAnglex.x = (charAnglex.x > 180) ? charAnglex.x-360 : charAnglex.x;
24.         charAnglex.x = Mathf.Clamp(charAnglex.x, -min_x, max_x);
25.         x_angle = charAnglex.x;
26.         gameObject.transform.rotation = Quaternion.Euler(charAnglex);
27.  
28.         Vector3 charAngley = gameObject.transform.eulerAngles;
29.         charAngley.y = (charAngley.y > 180) ? charAngley.y-360 : charAngley.y;
30.         charAngley.y = Mathf.Clamp(charAngley.y, -min_y, max_y);
31.         y_angle = charAngley.y;
32.         gameObject.transform.rotation = Quaternion.Euler(charAngley);
33.  
34.         if (target_compass < 20 ) {
35.         var rotation = Quaternion.LookRotation(target.position - transform.position);
36.         transform.rotation = Quaternion.Slerp(transform.rotation, rotation, Time.deltaTime * speed);
37.         }
38.         else {
39.             transform.rotation = Quaternion.LookRotation(main_part.transform.forward);
40.  
41.         }