var projectile : Rigidbody;
function Update ()
{
if (Input.GetButtonDown("Fire1"))
{
var clone : Rigidbody;
clone = Instantiate(projectile, transform.position, transform.rotation);
clone.AddForce(transform.TransformDirection (Vector3.forward * 10),ForceMode.Impulse);
//ForceMode.Impulse - добавляет импульс, а оно вам надо ?
}
}