tank-tower rotation works unexpected

hey guys! i would like to have a tank-tower controll like in world of tanks. that means, you have a tower ontop of your tank which can rotate around the y-axis. and you have a gun that rotates with the tower and on the z-axis i think.

(pillar has the script and gun is the verticalObj and suspension the horizontal) and here is my script:

 using UnityEngine;
 using System.Collections;
 
 public class WeaponMover : MonoBehaviour
 {
     public Transform target;
     public GameObject verticalRotateObj;
     public GameObject horizontalRotateObj;
 
     public float maxVerticalRotationSpeed = 2.0f;
     public float maxHorizontalRotationSpeed = 1.0f;
 
     public float maxVerticalRotation = 30.0f;
     public float maxHorizontalRotation = 135.0f;
 
     private Vector3 m_newHorEuler;
     private Vector3 m_newVerEuler;
     private Vector2 m_currPos;
     private Vector2 m_targetPos;
     private float m_conversion = 0.0f;
     private float m_minVertAngle;
     private float m_maxVertAngle;
     private float m_minHorAngle;
     private float m_maxHorAngle;
     private float m_targetHorAngle;
     private float m_targetVertAngle;
 
     // Use this for initialization
     void Start()
     {
         m_conversion = 180/Mathf.PI;
         m_newHorEuler = Vector3.zero;
         m_newVerEuler = Vector3.zero;
         m_currPos = Vector2.zero;
         m_targetPos = Vector2.zero;
     }
 
     // Update is called once per frame
     void Update()
     {
         m_currPos.x = horizontalRotateObj.transform.position.x;
         m_currPos.y = horizontalRotateObj.transform.position.z;
         m_targetPos.x = target.transform.position.x;
         m_targetPos.y = target.transform.position.z;
         m_targetHorAngle = AngleToTarget(m_currPos, m_targetPos);
         m_newHorEuler.y = m_targetHorAngle + 90.0f;
         horizontalRotateObj.transform.eulerAngles = m_newHorEuler;
 
         m_currPos.x = verticalRotateObj.transform.position.y;
         m_currPos.y = verticalRotateObj.transform.position.x;
         m_targetPos.x = target.transform.position.y;
         m_targetPos.y = target.transform.position.x;
         m_targetVertAngle = AngleToTarget(m_currPos, m_targetPos);
         m_newVerEuler.z = m_targetVertAngle + 180.0f;
          m_newVerEuler.y = m_newHorEuler.y;
         verticalRotateObj.transform.eulerAngles = m_newVerEuler;
     }
 
     private float AngleToTarget(Vector2 objectPos, Vector2 targetPos)
     {
         return (Mathf.Atan2(objectPos.x - targetPos.x, objectPos.y - targetPos.y) * m_conversion);
     }
 }

works ok but the problem is, if the target moves over a certain line, the gun gets flipped 180° on z-axis. i have no idea why... ;( every help is welcome!!!

update:

     void Update()
 {
         Vector3 angle = Quaternion.LookRotation(target.position - horizontalRotateObj.transform.position).eulerAngles;
         angle.x = horizontalRotateObj.transform.eulerAngles.x;
         angle.z = horizontalRotateObj.transform.eulerAngles.z;
         angle.y -= 90.0f;
         horizontalRotateObj.transform.eulerAngles = angle;
 
         angle = Quaternion.LookRotation(target.position - verticalRotateObj.transform.position).eulerAngles;
         angle.x = verticalRotateObj.transform.eulerAngles.x;
         angle.y -= 90.0f;
         verticalRotateObj.transform.eulerAngles = angle;
     }