Literally every forum for every current gen FPS game has people moaning about weapon balance and crying about things being overpowered. Please tell me more about your experience in developing AAA multiplayer video games and how you tackled the issue of weapon balance. Oh, wait, you're not a video game developer, you're just a sore loser who thinks the world owes them a living.
The weapon balance is fine. I've not used some of the weapons so I'm not qualified to discuss them in detail but the way I see it, they all do the intended job. Yes the DMR is powerful at long range, but it's supposed to be. Up close there are much better weapons. If you're playing on the big open maps, expect people to be using it and adapt accordingly. Stop sprinting everywhere, look at your radar, get behind cover, use your headset to co-ordinate with your team, choose the correct loadout for the situation and gametype. More importantly, accept that fact that it's not your God-given right to win every battle and every match. Yes it's competitive game, but moaning about weapon balance instead of asking how you can improve does nothing to help you win.
Or let's say I'm wrong and the DMR is overpowered. Then what? Nerf the DMR. Great, now the BR is overpowered. Nerf the BR. Now the AR is overpowered. Better nerf that too. So instead of using the money you paid for your copy of the game to pay their staff to put up new gametypes/maps/custom games settings/armour/specialisations/etc. 343 spend their time and money fixing weapon balance issues. So you start complaining that the maps are old, or that you want more armour. It never ends.
In short, stop complaining, either adapt or don't play. It's not like you don't have a DMR of your own.
(Not a personal attack on anyone in particular)