This is probably obvious to anyone that has played a Halo game after Halo: Combat Evolved, but if you haven’t played since then, or it’s just been a long time since you’ve played the others, then you may not be aware that the Scorpion tank controls were changed.

As the driver of a Scorpion in Halo 4 you can move the tank and shoot the main cannon but you cannot operate the machine gun turret. Only another player (or NPC) can shoot the machine gun.