Destroy clone when left Field of view?

I did this before but now I can’t remember how I did. I have a space shooter and for performance reasons I want to destroy bullets that travel out of the Camera’s visibility. Here’s the current code for my gun shooting if that helps.

    var bullet : Rigidbody;
var Spawn : Transform;
var shotspeed : int = 1000;
var offset = Vector3(0,0,3);
var firingRate : float = 0.1;
var lastFired : float = -100;

function Start ()
{
    gameObject.SetActiveRecursively(true);
}

function Update ()
{
   
 if (Time.time < lastFired + firingRate) {
return;
}
    if(Input.GetButton("Fire"))
    {
       Toss();
       
    }
}

function Toss()
{
    shot = Instantiate(bullet,Spawn.position+offset,Spawn.rotation);
    shot.rigidbody.AddForce(transform.forward * shotspeed);
     lastFired = Time.time;
}

Check if renderer.isVisible is false.