Carbine has always been 7 shot, but it is dead accurate, and you can fire it very quickly.
DMR can 5 shot, is accurate, but has the bloom so you have to pace your shots.
BR has been 4 shot, you can shoot them in succession quickly and be accurate. Now that its 5 shot and less effective at long range, it loses to the DMR unless your at mid/close range and even then its a toss up.
I feel if the BR was 4 shot and MAYBE slow its fire rate a hair, the weapons would all be balanced. Right now I feel Carbine and DMR are obviously better choices compared to BR.