How to rotate an instance relative to the parent and give it force?

I have a GameObject that rotates itself and has 4 guns that are children, I created the empty GameObject at the end of every gun and I would like to instantiate the bullet prefab the way the gun is rotated and give them force to go to that direction
Here is how the game looks: Imgur: The magic of the Internet
And this is my script to spawn, but don’t know how to give force:

public GameObject bullet;
 public Transform gun;
 public float time;
void Spawn()
 Instantiate(bullet, transform.position, gun.transform.rotation);
void Start()
 Invoke("Spawn", time);

You need to change bullet’s position in Update(). For example you could do this:

Change your instantiate code:

//Make an array to store all instantiated bullets.
// we'll need this to move each bullet in it's own direction later
private ArrayList bullets = new ArrayList ();

private void Spawn()
    //Store instantiated bullet as local variable 'clone'
GameObject clone = Instantiate (bullet, transform.position, gun.transform.rotation);
    //Add bullet to the list
bullets.Add (clone);
    //Call delayed method to delete bullet which is first in the bullets array
    // we need it to prevent performance issues when there's too much bullets in the scene
    // so we're adding 1 new bullet and deleting 1 old bullet
    Invoke ("DestroyBullet", 2f); //bullet will be destroyed in 2 seconds

private void DestroyBullet ()
    GameObject bulletToDestroy = bullets[0] as GameObject; //very first bullet in the array
    bullets.RemoveAt (0); //remove bullet from the array
    Destroy (bulletToDestroy); //destroy bullet's gameObject itself

Now in Update() you’ll change each bullet’s position:

void Update ()
     //Moving every bullet in the array relative to it's position and direction
     for (int i = 0; i < bullets.Count; i++) {
	GameObject bullet = bullets  *as GameObject;*
  •   bullet.transform.position =*

bullet.transform.position + bullet.transform.forward * Time.deltaTime;
//Time.deltaTime here
//is a multiplier for move speed. Now bullet will move for 1 meter per second.