Battle Rifle-Plasma Rifle

The BR-PR, which stands for Battle Rifle' and 'Plasma Rifle, can be seen in the Halo 2 level, 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. Sadly you cannot see it in the cutscene, for it can only be viewed when you pause the game and move the camera angle.

Reasoning
If you look closely at the Phantom,after and before the attack, you'll notice 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, meaning it would continue to fire, until it disappears in the mist. It has no guns and the BR-PR appears it will only fire for a brief moment. The Phantom fires in a three-round burst just 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.