So I started using the semi-auto attachment on my Assault Rifles (I have loved semi-auto weapons since the G3 in CoD4). I think I must have went around 10 straight games of TDM getting around 25+ kills and dying less than 4 times in each game.
The semi-auto attachment makes any, and every single assault rifle a two shot kill from medium to close range. For the tighter maps, I use it with the Honey Badger, and for long range, I use the Remington with a muzzle break for a considerably long 2 shot kill range. Yeah, the rate of fire sucks, but man... people drop like flies. You can easily pick up a whole team with one clip, whereas going full auto will definitely make you waste bullets and cause more reloads. I feel it's better than any of the Marksman rifles since you have the ability to use the VMR sight, which is such a boon for all ranges.
The only downside for me is that the Remington has a slow RoF and likes to jam up with I repeatedly press the trigger. You have to pace your button presses, because if you press it faster than the cycle of the bullets, the gun won't fire. This isn't an issue with the Honey Badger, though the 2 shot kill range for the Badger is quite a bit shorter, but at least it's silent. I guess the ARX160 would be a good choice due to it's fast RoF, but I don't like the laser sight which gives your position away too much.