instantiate a Rigidbody not work for me?

Hi, I see in script reference, it do it like this -- > http://unity3d.com/support/documentation/ScriptReference/Object.Instantiate.html

 using UnityEngine;
 using System.Collections;
 
 public class example : MonoBehaviour {
 
     public Rigidbody projectile;
     void Update() {
         if (Input.GetButtonDown("Fire1")) {
             Rigidbody clone;
             clone = Instantiate(projectile, transform.position, transform.rotation);
             clone.velocity = transform.TransformDirection(Vector3.forward * 10);
         }
     }
 }

and that is my script but doesn't work it. the prefab have attached a Rigidbody

 using UnityEngine;
 using System.Collections;
 
 public class Weapon : MonoBehaviour {
     public int power = 100;
     public int numBullets = 20;
     public int velocity = 20;
     public Rigidbody ammo;
     private Transform canon;
     
     void Start () {
         canon = transform.Find("CanonAK47");
     }
 
     void Update () {
         Rigidbody bullet;
         bullet =  Instantiate( ammo , transform.position,transform.rotation);
         bullet.velocity = transform.TransformDirection(Vector3.forward*velocity);
         Physics.IgnoreCollision( bullet.collider,transform.root.collider);
     }
 }

And the error is that -- >

Assets/MyScripts/Weapon.cs(35,17): error CS0266: Cannot implicitly convert type UnityEngine.Object' to UnityEngine.Rigidbody'. An explicit conversion exists (are you missing a cast?)

thanks for your help !!!!!