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' toUnityEngine.Rigidbody'. An explicit conversion exists (are you missing a cast?)

thanks for your help !!!!!