Machine gun help

I have a script for a machine gun but every time I press the fire button the gun keeps shooting and never stops even if there is no more bullets it just goes to minus. I have managed to change it to stop when I release the button but the gun shoots out all the ammo within a second. The machine gun should be more to shooting and stoping when button is help and released and shoot per half a second.

Please help me.

This is my script.

 public GameObject BulletMark;    

 public float shotInterval = 0.1f; // interval between shots
 public float reloadTime = 0.5f; // reload time
  public float FireRate;
 public float GunRecoil = 10;
 private float shotTime; // time control

 void Update () {
     if (Bullets>0 && Time.time> nextFire && Input.GetMouseButtonDown(0)){
         nextFire = Time.time + FireRate;
         InvokeRepeating("Shoot", 0.1f, 0.1f);                //Start Shooting
             //GunEffect.active = true;
         }else{
             if (Bullets==0 && AmmoLeft>0 && Input.GetKeyDown("r")){
                 Bullets = Bullets+8;
                 AmmoLeft = AmmoLeft-1;
                 AmmoClips.guiText.text = AmmoLeft.ToString();
                 Ammo.guiText.text = Bullets.ToString();
                 shotTime = Time.time + reloadTime; // set reload time
         }
     }
 }
 
 void Shoot(){
     animation.Play();
     audio.Play();
     //Normal Raycast                    //Add a random value to mouse position(r
         ray = Camera.main.ScreenPointToRay(Input.mousePosition+Random.insideUnitSphere*GunRecoil);
         if(Physics.Raycast(ray, out hit, 10)){
             //Declear new GameObject variable
             GameObject BulletHole;
             //Create the shootmark prefab at the hit point
             BulletHole = Instantiate (BulletMark, hit.point+(hit.normal*0.001f), transform.rotation) as GameObject;
             //Rotate the created object at the face of the normal LookAt (hit.point+hit.normal)
             BulletHole.transform.LookAt(hit.point+hit.normal);
             BulletHole.transform.parent=hit.transform;
             hit.transform.gameObject.SendMessage("Hit", 50, SendMessageOptions.DontRequireReceiver);
     
     }