Put some Debug.Log in that OnCollisionEnter function, make sure that it is definitely getting called.

well i put Debug.Log in and it got called for the missiles 3 times out of 5 but not for bullets

this is my gun script

var shotSound: AudioClip;
var bloodPrefab: GameObject;
var sparksPrefab: GameObject;
var loaded = 1;
var ammo = 0;
var startammo = 99999999;
var ableToFire = 1;
var muzzleflash : GameObject;
var dElay : float = 0;

function Start() {
ammo = startammo;
}

function Update () {
if (Input.GetButton("Fire1")){
if (loaded == 1){
if(ableToFire == 1){
if(ammo != -1){
        Invoke("Shoot",dElay);
    }
    }
    }
    }
    if (ammo == 0){
    ammo = -1;
    loaded = 0;
    Invoke("reload", 3);
    }
    }

    function Shoot(){
    ammo -= 1;
    ableToFire = 0;
    Instantiate(muzzleflash, transform.position, transform.rotation);
    Invoke("rechamber",0.12);
    if (shotSound) audio.PlayOneShot(shotSound);
    var hit: RaycastHit;
    if (Physics.Raycast(transform.position, transform.forward, hit)){
        var rot = Quaternion.FromToRotation(Vector3.up, hit.normal);
        if (hit.transform.tag == "Enemy"){
            if (bloodPrefab) Instantiate(bloodPrefab, hit.point, rot);
            hit.transform.SendMessage("ApplyDamage", SendMessageOptions.DontRequireReceiver);
        } else {
            if (sparksPrefab) Instantiate(sparksPrefab, hit.point, rot);
        }
    }
}

function reload() {
loaded = 1;
ammo = startammo;
}

function rechamber() {
ableToFire = 1;
}