I have always been a keen SMG user, and I have found that most of the combat situations I come across is Mid to Close ranged combat, on most maps.
The SMG has a very high rate of fire, 550 RPM to be exact, and yet with a big Ammo Capacity, it is hard to not hit an enemy, even at long ranges. For every shot you land on the head, it deals 75 damage, meaning you can kill an enemy in just 2 shots, and that equates to 0.21 seconds, shorter than anyone could react. It also does 29 damage on the Torso, meaning the enemy you hit would be dead in 4 shots, that is about 0.42 seconds, still faster than how most people could react. Most of the recoil could be compensated by crouching while shooting, and ensuring greatly improved accuracy. Considering it's fast speed of fire, it will kill most of the enemies you spot in the blink of an eye, long or short range, the enemy could be moving , but as long as you track the enemy's movements, it is likely you will do considerable damage to the enemy and in most scenarios, killing the enemy. Adding to that, SMG also have the ability to rip through enemy cover quickly.
Comparing to the rifle, it's rate of fire is 120 RPM, dealing 100 damage when shot on the head, killing the enemy immediately, yes, that might sound overpowered, but in actual fact, only about 50% of the shots I fire actually hit the enemy, it might be my crappy accuracy, but unless you are a aimpot smoker, or a robot like Tiki or any of those <@> members, you will miss a lot of your shots. Hitting the head is quite hard at both Long and short ranges. In the short range, it is hard to hit as the enemy is moving considerably faster as in Pixels per second, I suppose, and in the long range, it is just too small to hit the head. If you do not hit the head, you will likely to hit the Torso, which is 49 damage, killing the enemy in 3 shots or 1.5 second, that is a considerably longer time than the SMG. I won't say anything about the shotgun, as right now, it just sucks.
I think I have said enough for today, if you are still reading, I will shut the hell up now.