IEnumerator not looping correctly?

I have a pretty basic burst fire function for a gun- as in shooting three times in a row. Here is the script:

 void Update ()
     {
         timer += Time.deltaTime;
 
         if (Input.GetButton("Fire1") && timer >= fireRate && Time.timeScale != 0)
            {
                if (burstFire)
                   {
                     StartCoroutine(BurstShot());
                   }
             }
     }
 
  IEnumerator BurstShot()
     {

        for(int i = 0; i < burstSize; i++)

         {
             timer = 0f;

             Quaternion bulletRot = Quaternion.Euler(90, 0, 0);

             GameObject newBullet = Instantiate(bulletObj, transform.position, transform.rotation * bulletRot) as GameObject;

             newBullet.GetComponent<Rigidbody>().velocity = bulletSpeed * transform.forward;

             currentAmmo--;

             yield return new WaitForSeconds(burstRate);
         }
        
     }

The problem is, while the first projectile will shoot forward correctly, the next two projectiles will launch upwards in a near perfect arc. And for the life of me I can't figure out why. Why is everything in the coroutine repeating correctly except for the apparent rotation of the last two shots?