rocket Launcher GUI Text probleme

i got a rocket launche script attach to a GUI text that keep track of whats the max ammo and whats my ammo left but my ammo left in the GUI text does not go down as i shoot the misslie but it goes down in the inspector. what am i doing wrong????

here is my rocket launcher script:

var projectile : Rigidbody;
var initialSpeed = 20.0;
var reloadTime = 0.5;
var ammoCount = 5;
var maxAmmo =5;
var guiTextMissile : GUIText;
private var lastShot = -10.0;

guiTextMissile.text = ammoCount +"/" + maxAmmo;

function Fire () {
    // Did the time exceed the reload time?
if (Time.time > reloadTime + lastShot && ammoCount > 0) {
        // create a new projectile, use the same position and rotation as the Launcher.
        var instantiatedProjectile : Rigidbody = Instantiate (projectile, transform.position, transform.rotation);

        // Give it an initial forward velocity. The direction is along the z-axis of the missile launcher's transform.
        instantiatedProjectile.velocity = transform.TransformDirection(Vector3 (0, 0, initialSpeed));

        // Ignore collisions between the missile and the character controller
        Physics.IgnoreCollision(instantiatedProjectile.collider, transform.root.collider);

        lastShot = Time.time;
        ammoCount--;
    }
}

You have to update the text of the guiText component manually. Just copy the line where you initialize guiTextMissile.text to the line right after ammoCount--;