When I mean CQC, I mean shotgun and sub-machine guns. The P90 and the M1014 are good examples of this "class".

I'll cover shotgun first.
The shotgun was made for CQC. It hurts, it's powerful and it's mobile!
Tight situations are to your advantage, the shotgun being semi-automatic. Hit 'em with all you got.
During the match, suprise is your most powerful enemy, sneak around the large firefights and take out your enemies from behind.
High power pistols make a great backup for this gun!
Sawed-off shotguns are more mobile, but less accurate. So if you have a machine gun, sawed-off barrels are a great backup.
Keep low, this gives you some cover and a great firing positition when you kneel.
Have some time? Reload. Nothing is worse in CQC than hearing a click instead of a shot.
As a general rule, always watch both flanks and behind you, surprise works both ways.
As a last ditch effort to get a hit, charge 'em. Shooting them at point blank will have those BBs hurt like the devil!

The shotgun is one of my favorites, but it's time to cover the sub-machine guns!

SMGs do not have to be used in CQC, as they are much more accurate than the shotgun.
As with every weapon, element of surprise will have the enemy terrified the whole game. So keep low and go from cover to cover!
Don't go "Rambo". Please. Fire in bursts to keep accurate and conserve ammo.
Aim down the sights only when you need to, but always be on guard and alert.
Mini-Uzis are a good backup for this kind of gun because of the compact size and high rate of fire.
You can always hide, wait for someone and "snipe" the heck out of 'em. SMGs are easy to conceal and make a bit less noise in bursts.
Try to watch out for sniper fire, see if you hear it go by or the clang of the rifle, try to rat 'em out for some easy CQC action.

But, hey, at the end of the day, remember that its just a game and you should have fun doing it.

Thanks for reading!
Next in series: Teamwork strategies.