Don't call it a gun. In fact, don't even call it a rifle. It isn't even a weapon. It's a system. The MA5C Individual Combat Weapon System. If I didn't know better, I'd say that KP made that up himself when he penned the description of Halo 3's default spawn weapon, that hybrid of Halo 1's loveable bullet hose the Assault Rifle and Halo 2's pistol replacement the Battle Rifle.

One useful tidbit:

Fully shielded Spartans can take two blows from the rifle before dropping.

Granted, that's at close range, but that seems to indicate that under certain circumstances, the new AR might be just as, if not even more powerful, than previous "go-to" weapons.

UPDATE: For clarification, "two blows" mentioned above refers to melee strikes and not bullets fired, as the comparison to other weapons might seem to suggest. My bad.