Making a bullet destroy itself on contact.

Hi,

I have made a gun that will shoot bullets but I want those bullets to destroy themselves on contact with anything else.
Here is the code I use for shooting.

var BulletPrefab:Transform;
var force : float = 2000;

function Update()
{
if(Input.GetButtonDown("Fire1"))
{
var bullet = Instantiate(BulletPrefab, 

GameObject.Find("spawnPoint").transform.position, 

GameObject.Find("M4").transform.rotation);

bullet.rigidbody.AddForce(bullet.transform.forward * force);

}
}

What do I need to add in this script to make the bullet destroy itself?

Thanks

using UnityEngine;
using System.Collections;

/// <summary>    
/// This script is attached to the Blaster projectile and it   
/// governs the behaviour of the projectile.   
/// </summary>
  
public class BlasterScript : MonoBehaviour {
  
	//Variables Start___________________________________
	//The explosion effect is attached to this
	//in the inspector      	   
	public GameObject blasterExplosion;  
	//A quick reference.    
	private Transform myTransform;    	    
	//The projectiles flight speed.   
	private float projectileSpeed = 10;   	  
	//Prevent the projectile from causing  
	//further harm once it has hit something.   
	private bool expended = false;
   	//A ray projected in front of the projectile    
	//to see if it will hit a recognisable collider.    
	private RaycastHit hit;  
	//The range of that ray.   
	private float range = 1.5f;    
	//The life span of the projectile.    
	private float expireTime = 5;    	    	    	    
	//Variables End_____________________________________   
	// Use this for initialization   

    void Start ()    
	{  
		myTransform = transform;
		//As soon as the projectile is created start a countdown
		//to destroy it.  
		StartCoroutine(DestroyMyselfAfterSomeTime());

	}

	// Update is called once per frame
	void Update () 
	{
		//Translate the projectile in the up direction (the pointed
		//end of the projectile).   		
		myTransform.Translate(Vector3.up * projectileSpeed * Time.deltaTime);
		//If the ray hits something then execute this code.
		if(Physics.Raycast(myTransform.position,myTransform.up, out hit, range) && expended == false)
		{
			//If the collider has the tag of Floor then..
			if(hit.transform.tag == "Floor")
			{   
				expended = true;
				//Instantiate an explosion effect.
				Instantiate(blasterExplosion, hit.point, Quaternion.identity); 
				//Make the projectile become invisible.
				myTransform.renderer.enabled = false;
				//Destroy the projectile.    
				Destroy(myTransform.gameObject);   
			}   
		}   
	}
	IEnumerator DestroyMyselfAfterSomeTime()
	{
		//Wait for the timer to count up to the expireTime   
		//and then destroy the projectile. 
		yield return new WaitForSeconds(expireTime);
		Destroy(myTransform.gameObject);
	}    
}

That is the script I am using for my project. It is attached to the projectile GameObject.
I use another script to fire the projectile.