Battle Rifle-Plasma Rifle
From Halopedia, the Halo wiki
During the cutscene Uncomfortable Silence where the Arbiter is attacked by an Enforcer and is saved by a Phantom, the Phantom never really shoots. Instead there is a combined weapon that shoots the Enforcer, as seen in this image. You cannot see it in the cutscene; it can only be viewed when you pause the game and move the camera angle as shown in the below video link:
