If I may, here is the actual data for DMR, battle Rifle and Carbine. This is actual tag data, not guessed of keyframe data that is completely inaccurate.
So once you scan the information, you can clearly see that each has it's own strength and dominates the other in a certain area. To simplify the information further, I can list the jist of it here...
Battle Rifle: Fires the slowest, but fires the most shots per trigger pull, and for the amoun of projectiles it fires, it has resonable error constraints,
Carbine: Though it fires the fastest and deals the most damage, it has the largest error which makes connecting each shot more difficult than the others.
DMR: Medium firing speed, and low error make up for it's lowest damage dealing per shot.
Overall, the BR and DMR are about equal, except for it's massive range boost. Flat out, you can out range an opponent. But up close or before mid-range, the Carbine will eat you alive.