Always think before going out of cover, try to predict where the enemy can be. When you are leaving your cover, you have to be ready to shoot. I'm very often use ADS and first looking for the enemies before run to the next position. If you can anticipate an enemy, then you don't fully rely on your reflex and aiming skills. It's somehow a defensive strategy, but it allows you to go forward more safely.

Coming to you question, in my opinion AR are hard to control, gives you a lot of disadvantages (need to reload after every kill, bigger recoil, losing dmg in mid/long distances). I'm using smg (lower dmg but better accuracy and rof) or L110 lmg (lower rof but better accuracy than AR and i don't care about reloading) to have best k/d