Destroying bullets after certain time

I have a problem with destroying instances of a prefab.

I have searched the site and both of my solutions come from here but neither work properly. I am new to unity so any help will be greatly appreciated.

first of all the bullets are instantiated like this - this bit works fine

 Rigidbody newBullet = Instantiate(bulletPrefab, barrel.position, barrel.rotation) as  Rigidbody;
 newBullet.velocity = shooterT.forward * shootSpeed;

then in the script that is attached the bullet I have tried two different things’

void Start()
{
    Destroy(this, liveTime);
}

Which doesn’t seem to do anything

void Start()
{
    Destroy(gameObject, liveTime);
}

Which gives the error “The object of type “Rigidbody” has been destroyed but you are still trying to access it”

Changed it to Update and it works