LeftHand IK Target in weapon that is child of righthand?

I wanted to attach lefthand to my weapon, so i tried to use IK system. I made weapon be a child of righthand. and add leftHand IK Target to weapon. problem is, when the leftHand IK Target is not below rightSoulder, it works well. but when it is below rightShoulder, it works so weird. Why does this thing happen? and how to solve this problem?

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 [RequireComponent(typeof(AimSystem))]
 public class AimIK : MonoBehaviour {
 
     //Other component
     private Animator m_Ani;
     private AimSystem m_Aimsystem;
 
     private Transform m_AimTarget;
     public Transform m_AimPivot;       //This will make hands up and down according to direction player is looking.
     private Transform m_RightShoulder;
 
     //Grab Points
     public Transform m_LeftHandPosition;
     public Transform m_RightHandPosition;
     public float m_LeftHandIKWeight = 1f;
     public float m_RightHandIKWeight = 1f;
 
     public float m_LookWeight = 1;
     public float m_BodyWeight = 0.9f;
     public float m_HeadWeight = 1;
     public float m_EyesWeight = 1;
     public float m_ClampWeight = 1;
 
     // Use this for initialization
     void Awake () {
         m_Ani = GetComponent<Animator>();
         m_Aimsystem = GetComponent<AimSystem>();
         m_AimTarget = m_Aimsystem.m_RayTarget;
         m_RightShoulder = m_Ani.GetBoneTransform(HumanBodyBones.RightShoulder).transform;
     }
 
     private void OnAnimatorMove()
     {
         SetAimpivotPosition();
         SetAimpivotRotation();
     }
 
     private void OnAnimatorIK(int layerIndex)
     {
         //Hand IK
         if(m_LeftHandPosition)
         {
             m_Ani.SetIKPositionWeight(AvatarIKGoal.LeftHand, m_LeftHandIKWeight);
             m_Ani.SetIKPosition(AvatarIKGoal.LeftHand, m_LeftHandPosition.position);
             //m_Ani.SetIKRotationWeight(AvatarIKGoal.LeftHand, m_LeftHandIKWeight);
             //m_Ani.SetIKRotation(AvatarIKGoal.LeftHand, m_LeftHandPosition.rotation);
         }
 
         if (m_RightHandPosition)
         {
             m_Ani.SetIKPositionWeight(AvatarIKGoal.RightHand, m_RightHandIKWeight);
             m_Ani.SetIKPosition(AvatarIKGoal.RightHand, m_RightHandPosition.position);
             m_Ani.SetIKRotationWeight(AvatarIKGoal.RightHand, m_RightHandIKWeight);
             m_Ani.SetIKRotation(AvatarIKGoal.RightHand, m_RightHandPosition.rotation);
         }
 
         //Look IK
         m_Ani.SetLookAtPosition(m_AimTarget.position);
         m_Ani.SetLookAtWeight(m_LookWeight, m_BodyWeight, m_HeadWeight, m_EyesWeight ,m_ClampWeight);
     }
 
     public void SetHandsIKPosition(Transform _righthand, Transform _lefthand)
     {
         m_LeftHandPosition = _lefthand;
         m_RightHandPosition = _righthand;
     }
 
     public void SetAimpivotPosition()
     {
         m_AimPivot.position = m_RightShoulder.position;
     }
 
     public void SetAimpivotRotation()
     {
         //Weapon Direction IK
         Vector3 DestDir = m_AimTarget.position - m_AimPivot.position;
         Quaternion TargetRotation = Quaternion.LookRotation(DestDir);
 
         m_AimPivot.rotation = Quaternion.Lerp(m_AimPivot.rotation, TargetRotation, Time.deltaTime * 20);
     }
 }