Don't allow the player to shoot while projectile is travelling

Hi, I would like the player to be unable to shoot while the projectile is travelling. I've got collisions set up for when the bullet hits something. I know I can do this using booleans, but I'm not entirely sure how.

Gun Script:

 using UnityEngine;
 using System.Collections;
 
 public class ProjectileCS : MonoBehaviour 
 {
     //public variables
     public Rigidbody projectile;
     public float speed = 20.0f;
     public Rigidbody instantiateProjectile;
     
     // Update is called once per frame
     void Update () 
     {
         //input
         if (Input.GetButtonDown ("Fire1")) 
         {
             instantiateProjectile = Instantiate (projectile, transform.position, transform.rotation) as Rigidbody; //Object to clone, the position, the rotation
                 
             //Moving projectile
             instantiateProjectile.velocity = transform.TransformDirection (new Vector3 (0, 0, speed));
         }
     }
 }

Any help would be great.