wow wow wow... stop while your ahead.

The AR is a special weapon.  The only reason it was onboard the pillar of autom was because that ship was on a special mission.  Remember...(covies homewold, capture a prophet)  Anyway, if I understand correctly, the br was acually a standard issue gun for the UNSC and the AR was only for special missions.  Why, because mabye the AR wasn't mass produced, and the br was.  Im just going off the top of my head cause i don't have the books infront of me.  But don't worry, im sure they have their reasons for using that gun.  Remember, they know more about the story than us !