Placing Input.GetkeyDown() inside FixedUpdate() slows down the rate I can fire at

Hello all,

As the title suggests, if I place Input.GetKeyDown() inside FixedUpdate() to fire a Rigidbody, then the rate I can fire at becomes intermittent, no matter how fast I press the key. Now this makes sense as FixedUpdate updates less often than Update, but if this is the case then how can I work around it to shoot when I'd like?

Thanks

//----

 // Public variables are also shown in the Unity UI and can be changed in the Inspector
     public Rigidbody bulletPrefab;        // holds a reference to the bullet Rigidbody
 
     //    public Transform bulletPrefab;
 
     public float bulletPower = 1500f;        // used to set the power of the bullets
     //    public float casingPower = 1500f;        // used to set the power of the bullet casings expelled from side of gin
     //    public float moveSpeed = 2f;    
 
     // The general variable to hold the direction of the bullt - Enemy or Forward
     public Vector3 bulletDirection = Vector3.zero;
 
     // Variable to hold the direction of the enemy, assigned to the Instantiation, etc
     private Vector3 enemyDirection = Vector3.zero;
 
     // Used to reference the Player object
     private GameObject playerObj;
 
     // Used to reference the LockOnTriggerObject
     private GameObject lockOnTriggerObj;
 
     public float ShootDelay = 0.1F;
 
     private float Timer = 0.0F;
 
     // References the direction of the Player
     private Vector3 playerFwdDir = Vector3.zero;
 
     // Used to delay when the shooting begins after the Player is touched
     private bool delayFlag = true;
 
     // Used as a reference to check the PlayerTouch flag in MovementController
     private bool playerTouched;
 
     // Used as a reference to check the HoverFlag in MovementController script
     private bool hoverFlag;
 
     // The position of the spawner when hovering
     public Vector3 hoveringPosition = Vector3.zero;
 
     // The position of the spawner when running
     private Vector3 runningPosition = Vector3.zero;
 
     // Flag to let other scripts know if a bullet from the Right bullet_spawner has hit an object
     public bool R_Bullet = false;
 
     //-----------------------------------------------------
     void Start()
     {
         // Find the Player Object and then access its PlayerSounds script in order to player the sounds we need
         playerObj = GameObject.Find("Player");
 
         lockOnTriggerObj = GameObject.Find("PlayerLockOnTrigger");
 
         // The forward direction of the Player
         playerFwdDir = playerObj.transform.TransformDirection(Vector3.forward);
 
         hoveringPosition = new Vector3(4.28f, 8.35f, 4.66f);
 
         runningPosition = new Vector3(4.239313f, 7.397574f, 7.170765f);
     }
 
     //-----------------------------------------------------
     void Update()
     {
         // Gain access to the WeaponController script to check its playerTouched variable..
         playerTouched = playerObj.GetComponent<WeaponController>().playerTouched;
 
         if (playerTouched)
             Firing();
 
         // Check whether the Player is hovering or not. This is used so that we can re-position the 
         // BulletSpawner_R object to a position that appears to shoot out of the right gun.
         hoverFlag = playerObj.GetComponent<MovementController>().HoverFlag;
 
         if (hoverFlag)
             transform.localPosition = hoveringPosition;
         else
             transform.localPosition = runningPosition;
     }
 
     //-----------------------------------------------------
     void Firing()
     {
         // Access the CrosshairToggle script and check whether the we have an enemy targeted.
         // Ie. Has the ray we cast in the EnemyIntersect script hit an enemy, passed the 
         // Enemy's position to CrosshairToggle to place the crosshair onto the enemy.
         //            if (PlayerObj.GetComponent<CrosshairToggle>().EnemyTargetted)
         if (lockOnTriggerObj.GetComponent<ClosestEnemy>().EnemyTargetted)
         // && lockOnTriggerObj.GetComponent<ClosestEnemy>().EnemyDestroyed == false)
         {
             // Calculate the relative position of the Player to the Enemy ans store in bulletDirection
             bulletDirection = lockOnTriggerObj.GetComponent<ClosestEnemy>().EnemyPosition - transform.position;
         }
         else
         {
             bulletDirection = Vector3.forward;
         }
 
         // Make the Spawner rotation equal to the rotation of the bullet direction,
         // (towards an Enemy or straight ahead)
         transform.rotation = Quaternion.LookRotation(bulletDirection);
 
         // If touching the screen with LMB AND touching the Player is true..
         if (Input.GetMouseButton(0) && playerTouched)
         {
             Timer += Time.deltaTime;
 
             if (Timer >= ShootDelay)
             {
                 Timer = 0;
 
                 // Create a new variable named instance (of type Rigidbody - the bullet in this case) 
                 // Into this variable we are storing a reference to the creation of a new object that is of type Rigidbody (bullet).
                 // Requires three pieces of information namely, Instantiate(What to make, Where to make it, a rotation to give it);
                 Rigidbody newBullet = Instantiate(bulletPrefab, transform.position, transform.rotation) as Rigidbody;
 
                 //Rename the bullet instances to avoid the default '(Clone)' extension which Unity adds.
                 newBullet.name = "bullet";
 
                 /* we will apply the force by first referring to our variable that represents the newly created object—instance, 
                 then using the AddForce() command to add a force in the direction of the fwd variable—multiplied by the
                 public variable named power that we created earlier. */
                 newBullet.AddForce(transform.forward * bulletPower);
             }
         }
     }
 
     //-----------------------------------------------------
     // DELAYER
     // Used to delay the shooting of the bullets to give the animation enough time to raise the arm, etc
     IEnumerator Delayer()
     {
         if (delayFlag)
         {
             yield return new WaitForSeconds(0.2f);
         }
 
         delayFlag = false;
     }