The SMR assault rifle is the hardest weapon to use in Call of Duty Black Ops 2 I have found so far. It is basically an inferior version of the FAL DSW (click for a guide for that weapon) as the SMR is semi automatic with a slower fire rate, more recoil but higher damage than the FAL DSW. It is a very passive rifle to say the least. This guide explains how to use the SMR well and what the best class setup is.
There is a YouTube video with a visual version of this guide at the end of this guide.
The Select Fire attachment is very good with the SMR since it negates two downfalls the semi automatic rifles have: when you pull the trigger manually you are going to mess with your aim sometimes and you may not be able to reach the fire rate cap as you can pull the trigger both too fast and too slow both of which reduce your fire rate. Both these problems go away with automatic fire.
The SMR needs to be used very passively. You are going to loose to every assault rifle and sub machine gun at close and medium range (as well as shotguns). You are much better at long range, but snipers beat you there as you can’t one shot enemies. So you will have to stay back behind cover and pick people off from a longer range.
The class I recommend is the following:
Because you are going be staying back in cover you need perks to help keep you there. When you kill an enemy they will know where you are and tend to throw grenades and tactical equipment at you to try and get a revenge kill. To stop them from killing you use Flack Jacket to reduce the impact of explosives and Tactical Mask to reduce the impact of concussion and flash grenades as well as shock charges. Ghost will help you stay hidden even when the enemy has a UAV up. Because you are going to aiming down the sights and you may get shot at Toughness reduces the flinch that otherwise might cause you to miss your target.
Since you are staying back behind cover you don’t need any equipment or secondary. If you run dry in the SMR duck and reload and pop back up.
The attachments are there to make the SMR easier to use. The recoil will cause you to loose enemies in the iron sights at longer ranges, so an optical sight is going to help you adjust your aim to counteract the recoil (which one is up to you, although the Reflex Sight is good as it doesn’t reduce your peripheral vision too badly. Select Fire was already discussed and the Fore Grip is a very solid choice since it reduces the SMR’s high recoil.