It's even explained in the books that MC's suit interfaces with contacts on all of the weapons, which allows his HUD to display that information. Even going so far as to suggest that the zoomed "scopes" that you use aren't actually physically being used, but are simply a different view mode of his visor which is compatible with that weapon, hence the camera-zoom sound effect. As far as I know, the pistol in Halo didn't actually have a scope on it. Just a small camera/sight on the top that interfaced with Mjolnir armor and those little holo-displays on the Marines' helmets.