Input.GetButton Method

Gun.js

var bulletPrefab : GameObject;
var initialVelocity : float;

function Update () {
	if (Input.GetButton("Fire1")) {
		var screenPoint = Input.mousePosition;
		screenPoint.z = 10.0;
		Instantiate(bulletPrefab, transform.position, transform.rotation).rigidbody.velocity
		= ((camera.ScreenToWorldPoint(screenPoint)) - transform.position).normalized * initialVelocity;
	}
}