Having trouble with instantiating bullets at multiple points via a list of transforms.

Hi, I am having trouble with Instantiating bullets at six different transforms that are stored in a List of transforms. It keeps throwing the error (NullReferenceException: Object reference not set to an instance of an object Gun.Shoot() (at Assets/Scripts/Gun.cs:21). I already threw in some Debug.Logs to see if the points I put in the list were null and they weren't so I am confused, I am also a beginner so that might be why I am stuck on this simple issue.

Here is the code:

Bullet.cs

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class Bullet : MonoBehaviour {
     public float bulletVelocity = 100f;
 
     public List<Gun> gunSpots = new List<Gun> ();
     [HideInInspector]
     public List<Transform> muzzleSpots = new List<Transform>();
 
     void Start(){
         FindGunPositions ();
 
     }
 
     public void FindGunPositions(){
         foreach (Gun gun in gunSpots) {
             muzzleSpots.Add (gun.transform);
             //Just checking that the gun positions were not null
             //Debug.Log (gun.transform.position);
         }
     }
 
     void Update(){
         
     }
     
 
 }

Gun.cs

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class Gun : MonoBehaviour {
 
     public Bullet bulletPrefab;
     Bullet bulletInstance;
     Bullet bulletClass;
 
     void Start () {
         bulletClass = GetComponent<Bullet> ();
     }
     
     void Update () {
         Shoot ();
     }
 
     void Shoot(){
         if (Input.GetButton ("Fire1")) {
             foreach (Transform gun in bulletClass.muzzleSpots) {
                 Debug.Log (gun.transform.position);
                 bulletInstance = Instantiate (bulletPrefab, gun.position, Quaternion.identity) as Bullet;
                 bulletInstance.transform.position = bulletInstance.transform.position * Time.deltaTime * bulletClass.bulletVelocity;
             }
         }
     }
 }

Thank you for the help in advance!