Weapon Sway Script messes up weapon rotation

So I'm in the process of learning Unity 5 and scripting in it. One thing I wanted to give a try was to make a simple weapon sway script like you see in games such as battlefield or csgo. After a bit of documentation I finally got a script going (see below).

 using UnityEngine;
 using System.Collections;
 
 public class WeaponSway : MonoBehaviour {
 
     float mouseX;
     float mouseY;
     float movementX;
     float movementY;
     float smooth = 3;
     public float speed;
 
     Vector3 defaultposition;
     Vector3 newPos;
 
     // Use this for initialization
     void Start () {
 
         defaultposition = transform.localPosition;
     }
     
     // Update is called once per frame
     void Update () {
 
         mouseX = Input.GetAxis("Mouse X");
         mouseY = Input.GetAxis("Mouse Y");
 
         movementX = mouseX * Time.deltaTime * speed;
         movementY = mouseY * Time.deltaTime * speed;
 
         Quaternion final = Quaternion.Euler(defaultposition.x * movementX, defaultposition.y * movementY, 0);
         transform.localRotation = Quaternion.Slerp(transform.localRotation,final,Time.deltaTime*smooth);
 
     }
 }

However, for some reason the simple model of the weapon I'm using changes rotation and the weapon doesn't sway like intended. I can't see the problem in the script.

Here is a gif of what is happening: http://imgur.com/zi6w6Om