I think the BR should get a slight damage buff so that it pops shields 3 and 2/3 bursts. This would make it a potential 4 shot kill at close medium range which would emphasize its role against the dmr.
That would be the best option. Principally I feel like it should have more of a long range feel and shouldnt function as well as it does up close, but no idea how they would go about doing that. It still would/should still fire faster than the LR and BR.
I still think the AR would be viable, regardless of the buff. If you're using the AR right, you're going to be using the easier aim to out maneuver precision weapons up close that very slightly increasing the BR's damage won't really matter. This buff would make it a relatively rare 4 shot kill.
As it is now, I'd consider buffing the Storm rifle and the suppressor to be more on par with the AR.