The DMR has several advantages over the Carbine and BR, like only 5 actual bullets needed to land a kill or the insane magnetism, or the fact that shooting someone to death at any range besides point blank including the range of the BR and Carbine is easy.
My proposed buff to the other two rifles is just to make them have the same range as the DMR. I honestly think the DMR will still have the advantage but at least it will be on even range ground. I used the Halo 2 carbine the most out of the versions of the rifle and what made it so good in halo 2 was it's firing speed and the same range as the BR. It fired EVEN faster then the halo 4 version but more importantly it had the range to back it up at a cost of slightly low accuracy at range rapid shooting. The BR will still be balanced except those frustrating times you decided to use a BR in big team, your getting picked off by double its range by the DMR.
Anyway, I thought long and hard about how to solve this "DMR is better then the other rifles" problem. And don't kid yourself if you think the other rifles are fine.