Instantiated object rotation abruptly.

I have an empty gameobject called shotspawn at which I am instantiating a missile from the prefab.

I am using this code to instantiate-

 Instantiate (shot2, shotSpawn.position, shotSpawn.rotation);

the shot2 which is instantiated above has a script that says-

 //to make the missile move forward
     rigidbody.velocity = transform.forward * speed;

The shot2 has rotation set to 0 and it has only one child having rotation 0 too. this child has objects like mesh and particles having some rotation set.

THE PROBLEM- when the shot2 gets instantiated, it moves forward but is rotating on it's own axis. :(

it doesn't rotate when it is already present in the scene but on instantiation, it keeps on rotating while moving forward.