Create Bullethole with onCollisionEnter

Where is this script attached to? Is the collider a trigger?

So you check if the bullet collides with a bullet?

But like that

 if(col.gameObject.name == "Bullet1")

you test if the hitted collider is a bullet1. How about asking if it hits a wall?

It is your collider thing.

you need to make your bullet cast the ray in the update or fixedupdate and test if it hits a collider.

 void Update ()
     {
         RaycastHit hit;
         Ray ray = new Ray(transform.position, transform.forward);
         if(Physics.Raycast(ray, out hit, 100f))
         {
             if(hit.collider.tag == "walls")
             {
                 Instantiate(bulletHole, hit.point, Quaternion.FromToRotation(Vector3.up, hit.normal));
             }
         }
     }

Test this out.

You're welcome. I just could write this as an answer, but i don't think it useful.