The DMR in my opinion is not too overpowered, because if you strafe good enough you can seriously throw off a DMR. But the DMR is optomized for long range, not everything. Which is why the BR is slightly underpowerd.
The BR is easy to control at close, medium, and semi long ranges. But the DMR can be used as effectively (sometimes even more effectively) than the BR at all ranges, and it has almost no limit to its range.
The DMR should only be worth using at longer ranges, but can still have a chance at closer ranges, but just doesn’t do it as good as the BR.
The DMR should kill as fast as a BR user that misses two bullets (making it a 15 shot kill instead of a 13) making the BR better suited for its optimum range which is medium range.
The other thing you could do do the DMR is give it less bullet magnetism too, making you have to pace your shots at long range in BTB.
All these things would make all the primary weapons balanced. (no need to change the carbine or light rifle if the DMR is nerfed)