If you have door animations then use for the button something like

var door : GameObject;

function HitByBullet() {
 door.animation.play("open")
}

When you determine that the button has been hit, call the HitByBullet function.

If you haven't figured out how to tell if the button was hit yet, check the scripting reference for OnTriggerEnter, OnCollisionEnter, and Physics.Raycast; one of those should make you happy.