Help with ammo script :)

Ok so i am a bit of a noob when it comes to coding, but i made this ammo script,
however when ever i shoot the number does not go down, can anybody help?

var Ammo = 20;
var MaxAmmo = 20;
var AmmoPool = 100;
if (Input.GetButtonDown ("Fire1")) {
Ammo--;
}

if (Input.GetButtonDown ("r")) {
Ammo++;
AmmoPool--;
}



function OnGUI () {
 GUI.Box( Rect(10,10,100,90), Ammo.ToString() );
 GUI.Box( Rect(40,10,100,90), AmmoPool.ToString() );
}

This will start you going.

private var ammo_mag:int = 5;
private var ammo_all:int = 10;

function Update(){
	if (Input.GetButtonDown ("Fire1")){
	    if(ammo_mag > 0)ammo_mag--;
	}

	if (Input.GetKeyDown(KeyCode.R)) {
	    Reload();
	}
}

private function Reload(){
    if(ammo_mag == 0){
        if(ammo_all > 0){
            ammo_all -= 5;
            ammo_mag = 5;
        }
    }
}

function OnGUI () {
    GUI.Box( Rect(10,10,70,25), ammo_mag.ToString() +"/"+ammo_all.ToString() );
}