How could i make my gun move where my cursor is?

Edit: After getting more specs I've re-done this answer.

There are 2 scripts, The CameraScript and the PointerScript. The CameraScript needs a reference to a PointerScript. And the PointerScript must be attached to a gameobject that is childed to another gameobject. Their world positions need to be the same. When the PointerScript is selected a frustum gizmo will be drawn that shows the approximate field of view the PointerScript can take

CameraScript

     using System.Collections;
     using System.Collections.Generic;
     using UnityEngine;

     [RequireComponent(typeof(Camera))]
     public class CameraScript : MonoBehaviour
     {
       public PointerScript myPointer;

       private Camera myCamera;
       private RaycastHit rayHit;
       public Vector3? CurrentPoint { get; private set; }

       private void Awake()
       {
         myCamera = GetComponent<Camera>();
       }

       // Use this for initialization
       void Start ()
       {
         if (myPointer == null)
         {
           Debug.LogError(string.Format("'myPointer' was not set, disabling CameraScript on gameobject: ({0})", name));
           this.enabled = false;
         }
         }
 
         // Update is called once per frame
         void Update ()
       {
         CurrentPoint = GetNewPosition();
         myPointer.UpdateTarget(CurrentPoint);
       }

       private Vector3? GetNewPosition()
       {
         Ray myRay = myCamera.ScreenPointToRay(Input.mousePosition);

         if (Physics.Raycast(myRay, out rayHit))
         {
           return rayHit.point;
         }
         return null;
       }
     }

PointerScript

     using System;
     using System.Collections;
     using System.Collections.Generic;
     using UnityEngine;

     public class PointerScript : MonoBehaviour
     {
       private Transform tran;
       private Transform parentTran;

       [Tooltip("Maximum degrees this transform will look either side (along the y-axis) of its parents forward")]
       public float yawConeDegrees;

       [Tooltip("Maximum degrees this transform will look up and down of its parents forward")]
       public float pitchConeDegrees;

       private Vector3? currentTarget;
       private Vector3 aimDirection;
       private float returnTime;

       private void Awake()
       {
         tran = transform;
       }
  
       void Start () {
         if (tran.parent == null)
         {
           Debug.LogError(string.Format("This script 'PointerScript' requires a parent, disabling PointerScript on gameobject: ({0})", name));
           this.enabled = false;
         }

         parentTran = tran.parent;
         }
  
       void Update ()
       {
         if (currentTarget.HasValue) // If we have a position, rotate towards it
         {
           aimDirection = (currentTarget.Value - tran.position).normalized;
           Vector3 parentForward = parentTran.forward;

           Vector3 yawFlat = aimDirection;
           yawFlat.y = 0;
           yawFlat = yawFlat.normalized;

           Vector3 pitchFlat = aimDirection;
           pitchFlat.x = 0;
           pitchFlat = pitchFlat.normalized;

           float yaw = Vector3.SignedAngle(parentForward, yawFlat, tran.up);
           float pitch = Vector3.SignedAngle(parentForward, pitchFlat, tran.forward);

           if (Mathf.Abs(yaw) <= yawConeDegrees && Mathf.Abs(pitch) <= pitchConeDegrees)
           {
             tran.LookAt(currentTarget.Value, parentTran.up);
           }
         }
         }

       private void OnDrawGizmosSelected()
       {
         if (transform.parent != null)
         {
           Matrix4x4 previous = Gizmos.matrix;
           Gizmos.matrix = transform.parent.localToWorldMatrix;
           Gizmos.DrawFrustum(Vector3.zero,pitchConeDegrees*2, 5,0, yawConeDegrees / pitchConeDegrees);
           Gizmos.matrix = previous;
         }
 
       }

       public void UpdateTarget(Vector3? newPoint)
       {
         currentTarget = newPoint;
       }
     }