Cannonball not shooting according to the rotation of the cannon.

Hello, im working on a small assessment piece and am having trouble with getting my cannon to shoot the cannon ball.

I have the cannon (very basic model) and a cannonball prefab (Rectangle) and i have made the cannonball shoot according to an empty gameobject that i have attached to the cannon via a fixed joint.

and it shoots fine. But i cant seem to figure out how to it to shoot according to its rotation.

Heres the code ive got.

Thanks for any help in advance.

 public class Shooting : MonoBehaviour 
 {
 
     public Rigidbody thePrefab;
     public float speed = 5.0F;
     
     void Update ()
     {
         if(Input.GetButtonUp("Jump"))
         {
             Rigidbody instance = Instantiate(thePrefab, transform.position, transform.rotation)as Rigidbody;
             instance.AddForce(new Vector3(500 * speed,0,0));
         }
     }
 }