I had a working script, then somewhere along the line something got deleted and I cannot figure out how to get back to where is was. I have a driveable tank with a game object attached to the end of the barrel that fires projectiles on a left mouse click. With the following script however, my bullets spawn but dont move. What have I miscalculated?

using UnityEngine;
using System.Collections;

public class fireBullet : MonoBehaviour {

    public Rigidbody bullet;
    public float cannonPower;

    // Use this for initialization
    void Start () {

    }

    // Update is called once per frame
    void Update () {
       //fire cannon
       if(Input.GetButtonUp("Fire1")){

       //create projectile at location script is attached to
         Rigidbody instance = Instantiate(bullet, transform.position, transform.rotation) as Rigidbody;

       //add force to projectile
         Vector3 fwd = transform.TransformDirection(Vector3.forward);
         instance.AddForce(fwd*cannonPower);

       }
    }
}