Instantiate object that points to the direction

I'm trying instantiate a object from other (a Cannon), and this Cannon have rotation and point for some angle. And when I press a button the "projectile" need go to the same direction that Cannon pointed!

I can rotate the object and instantiate it for the same rotation that Cannon, but I cannt make this object go to the same direction of the cannon point.

• • little piece of CODE:

var vec_power_shot = Vector3.zero;

var speed_human = 300;

var power_shot = 7;

//When I press Fire

function HumanCannon()

{

vec_power_shot.x = speed_human;

vec_power_shot = transform.TransformDirection(vec_power_shot);

var instanceShot = Instantiate(objtPlayer,transform.position,transform.rotation);

instanceShot.rigidbody.AddRelativeForce(-vec_power_shot);

//negative because turn more correctly the direction, and because I think my Canon is with different directions

}

I hope you understand what I want, at least what I said. =]