Adding Ammo to make gun loose/add ammo as used/picked up

Just make a new guiText (a gameObject with a guiText component). Make a variable in your script: public GameObject guiTextObject and put your guiText object in that slot in the editor. Then in your script just use guiTextObject.guiText.text = bulletsLeft.ToString();

 public GameObject guiTextObject;
 
 void MachineGun_Fire(){
 
     if (bulletsLeft <= 0)
     {
         StartCoroutine("reload");
  
         return;
     }
     // If there is more than one bullet between the last and this frame
     // Reset the nextFireTime
     if (Time.time - fireRate > nextFireTime)
         nextFireTime = Time.time - Time.deltaTime;
  
     // Keep firing until we used up the fire time
     while (nextFireTime < Time.time)
     {
         switch (typeOfBullet)
         {
             case BulletType.Physical:
                 StartCoroutine("FireOneShot");  // fire a physical bullet
                 break;
             case BulletType.Raycast:
                 StartCoroutine("FireOneRay");  // fire a raycast.... change to FireOneRay
                 break;
             default:
                 Debug.Log("error in bullet type");
                 break;
         }
         shotsFired++;
         bulletsLeft--;
         guiTexObject.guiText.text = bulletsLeft.ToString();
         nextFireTime += fireRate;
         EjectShell();
         Kick();
     }
  
 }

Now all you have to do is make a new guiText (GameObject -> Create Other -> GuiText), position it how you want, and then select the object that contains your gun scripIn the editor drag the new guiText object to where it says guiTextObject in the inspector. If you don't know how to do that you probably need to watch some more basic Unity tutorials. Here's more info on guiTexts: http://docs.unity3d.com/Documentation/Components/class-GuiText.html