I'm starting to come along in Modern Warfare 3. A big part of my game up to this point has been submachine guns, or SMGs. These guns are notable for being highly mobile automatic weapons. They lack the range to match up against some of the other weapons and can pale in comparison to the power of shotguns at short-range. However, the best reason for me to pick up SMGs are their versatility. They are good at mid-range, good in close quarters, good firing off the hip, and good when ADS. You may have to be somewhat creative to come up with a good class that improves the performance of these weapons, yet I feel they are fairly good weapons to use for beginners and experts alike. If you're looking for some SMG classes to try, you are more than welcome to try some of mine.
This class is built to move quickly and reduce the chance of unwanted surprises. I use my Mobile class primarily on smaller maps such as Dome, Hardhat, and Piazza. The UMP45 is my weapon of choice, so much so that it is the only golden gun I have at the time of writing. With the addition of Rapid Fire and the Range proficiency, the UMP45 becomes very good for quick kills and doesn't lose its power at mid-range. Extreme Conditioning and the Juiced deathstreak allow me to move around the map quickly. Adding Assassin I don't have to worry about UAVs while running after a kill or two and Steady Aim Pro makes me ready to fight faster once I find someone. The pistol is there for quick finishes or melee kills, while the grenades and killstreaks are rather arbitrary.
This swift, silent assassin class is used by me mostly on larger maps to get the jump on snipers or in game types like Free-for-all or Kill Confirmed that demand the ability to kill at close range while not giving away your position to opponents. The PP90M1 with its rapid fire capabilities seems like a good fit for a Silencer, and the inclusion of the Melee proficiency makes it outstanding for getting your kills silently. The MP9 with a Red Dot Sight provides a ranged option at the risk of giving away your position. The Throwing Knife gives its user a silent instant kill option. The perks are all set up for stealth: Blind Eye to keep out of sight of copters, Assassin to prevent yourself being seen by UAVs, and Dead Silence not only to move in quietly on an opponent but also to reduce the effects of other recon methods. The Specialist killstreaks improve the mobility and close range abilities of this class, much like the above option. Hardline is there simply to get to All-Perks Bonus faster and can be swapped out for Blast Shield to prevent your streak from being ended by a stray grenade, Sleight of Hand to offset the slow reload time of the PP90M1, or Scavenger so you don't have to pick up another possibly unsuppressed gun in the middle of your streak.
All in all, I advise newer players to have options in your game. Weapons like shotguns and sniper rifles that are very good at one thing are good when you know how to use them properly. When you are inexperienced, it is better to be able to handle multiple situations. You might be surprised how valuable options are on the battlefield.
ArgentStew, the Gaming Sage