How can I add rotations to transform.rotation?

So, I'm trying to make an FPS game with Instantiate as a bullet method. I want the bullet to rotate either 90 or -90 degrees(I haven't determined which) across the gun's rotation. (i.e. if the gun is at 0, 90, 0, I want the bullet to spawn at 0, 0, 0, or 0, 180, 0, depending on which it is.) I've tried multiplying transform.rotate by Quaternion.Euler(0, -90, 0) , but no matter what the y is the bullet rotates to face the wrong way, but 90 degrees. Here's the code:

 using UnityEngine;
 using System.Collections;
 
 public class FiringScript : MonoBehaviour {
 
     Animator anim;
     public GameObject Bullet;
     public float BulletSpeed = 100.0f;
 
 
     // Use this for initialization
     void Start () {
         anim = GetComponent<Animator> ();
     }
 
     // Update is called once per frame
     void Update () {
 
         if (Input.GetKeyDown ("mouse 0")) {
             anim.SetBool ("Recoiling", true);
             GameObject Bullet2 = Instantiate(Bullet, transform.position, transform.rotation * Quaternion.Euler(0, -90, 0)) as GameObject;
         } else {
             anim.SetBool ("Recoiling", false);
         }
     }
 }
 

Any suggestions?