ACR w/ RDS is one of the most used primaries in competitive play. It really is so accurate and due to the fire rate, it drops people quickly. Remember to use Scavenger though as you may run out of bullets after about 7 or 8 kills without it.
acog and/or silencer. red dot does good but i like to see what im shooting from far on large maps. i just love how far it can shoot and it makes me dislike using sub mchine guns even more. i only use holo on highly accurate guns like m4a1.