MP7 w/long barrel and RDS lightweight coldblooded ex con+engineer FHJ EMPs or Blackhats x2
The long barrel and RDS make it deadly from range and the perks and gear allow you to sprint all over the map constantly without fear of equipment or snipers since you can see where theyre all posted up. Tom me, SMGs are about constant movement.
focus upon starting to fire THEN adsing, don't ads then fire.
Focus burst fire, except when in extremely close quarters, don't try to outgun AR's / Snipers at anything beyond a medium range.
Never. Use. Rapid. Fire.
Don't stop moving, learn to use Strafing to help correct aim whilst making you a more difficult target.
Start aiming for the chest, let the natural recoil guide bullets into the enemies chest / head.
Be mobile, don't stop moving, everytime you move into a room check every angle out of the corners of your eyes, moving into a corridor ads pre-emptively down where the corridor will be.
best place to practise? hardcore! it tones your reflexes, teaches you the kill angles, and trains you to check every single nook and cranny, plus the reduces HP pools make the smaller, faster weapons more useful, once you're in that mindset, take it back to Core and you'll notice a real improvement.
and remember, when you see those deathcams, that perfectly steady accuracy isn't real, the game only records misses / hits and whereas the gun probably kicked everywhere in actuality, the killcam will show an eerily steady smg spread, it's not real.
I second what these guys have already said. Out in an open area you're a target though as you won't be able to engage anything at mid to long range with any amount of success. Corridors and interiors are where you have an advantage over everything except shotguns. I've found sensor grenades work great with my SMG class, as I toss them in my blind spots as I run through the map.