Battle Rifle-Plasma Rifle

The BR-PR, which stands for Battle Rifle and Plasma Rifle can be seen in the Halo 2 level Sacred Icon, during the cutscene Uncomfortable Silence where the Arbiter is attacked by an Enforcer and is saved by a Phantom. However the Phantom never really shoots. Instead there is a combined weapon that shoots the Enforcer, as seen in this image. However you cannot see it in the cutscene; as it can only be viewed when you pause the game and move the camera angle.

Reasoning
If you look closely at the Phantom, both after and before the attack, you'll see that it has no cannons on it. If the Phantom were to shoot the Enforcer by itself, it wouldn't stop shooting until the Enforcer was down, which meant that it would fire and fire, until it disappears in the mist. Now, when it has no guns and the BR-PR appears, it will only fire for a brief moment. It also fires in a three-round burst like the Battle Rifle.

Trivia

 * This is similar to the Scarab Gun, meaning that if you could pick up the BR-PR, it would be akin to a Phantom Gun of sorts.