I don’t see any reason to nerf it. 343 did very well at making the Carbine, BR and DMR a Rock, Paper, Scissors game.
From my experience of using and fighting with it, it comes down simply to this: DMR is better at long range, Battle Rifle is better at close range. I don’t often get beat in a BR vs DMR duel at close range when wielding the BR. There’s nothing to nerf about it. And at long range, with the BRs spread, it’s hard to take down a DMR wielder due to them only having to deal with the bloom.
There’s a few things to note about why which is stronger. First, the BR tends to cause more bounce when shooting someone in scope. I think it has a lot to do with bullet impact, and the BR impacts with 3 bullets and while using a DMR, I tend to notice I go up a bit more. This isn’t much to deal with at long range as it’s unlikely all 3 bullets will hit, but at close to mid, it makes it impracticable to aim through scope. The BR doesn’t experience as much of a problem with this as I’ve seen, and the BR also allows you to use it’s inert spread to your advantage, being able to cover in a small cone rather than just one point as with the DMR. Again, this gives you a close to mid advantage that dwindles at long range.
The BR and DMR take exactly 5 headshots each to kill a target, so nerfing damage would just be unfair if one side was nerfed and the other was not.
The Carbine doesn’t have a spread and it doesn’t deal with much bloom. It acts like the DMR but with it’s increased firing rate, it can be used in a similar fashion as the BR. However, to make it not incredibly overpowered, it also takes (I believe) 8 headshots to drop a target. These 8 can be shot off in about the same time as 5 from the DMR/BR, again, because it fires faster, but it takes a lot of concentration to do such, making it weaker than it’s UNSC counterparts at their respective ranges.
What I just said was basically, in Close Quarters Combat, the Battle Rifle would beat the DMR and Carbine. At Long Ranged Combat, the DMR would beat the Battle Rifle and Carbine. However note that the Carbine, while not being the best at either, is worst at neither. The Carbine would beat the DMR at close range and would beat the BR at long range. This makes the Carbine a flexible weapon and allows you to get close and at far, but is superior at neither range, making you still have to be careful. At mid, they all seem to meet at the same point, making them rather balanced.
If you want to talk overpowered, let’s talk the Light Rifle. 
It has no bloom, recoil or spread. It also takes 5 headshots to kill a target but doesn’t have quite the firing speed of a Carbine. It also doesn’t allow you to use the cone tactic like the BR and the crosshair is intentionally made to make it slightly difficult to make a perfect shot at long range. I would say the light rifle dominates at mid range, but can still be beat at close and long by the DMR and BR. Note however, the the Light Rifle is a forerunner weapon and is kind of a top-tier multiplayer weaopn, so this advantage is most likely intentional. But again, it’s not unbeatable, even in the best of hands and still has it’s downfalls. When getting directly assaulted, I’d rather have a carbine. But while doing the assaulting, I’d probably rather have a Light Rifle.
All in all, I don’t think anything needs nerfing. 343 actually did an amazing job balancing the weapons and managing to keep them all in game to make everyone happy. I’d go as far to say as they did better than Bungie at balancing, though do note that their work on doing such is based on 10 years of Bungie working to get it right. They just iced the cake.
