Well if you just want to know what to use...
CQC: M4A1 is really smex for CQC. I nearly always get a multi-kill every time I spawn when I'm in OMA. But if you're ranked highly enough and you're willing to spend 1.5k GP on a really smex gun, get the SG550 + the really big magazine. Total smex machine.
Medium range: Once again, the SG550. You could also use the FAMAS, which has a remarkable ability to always hit your enemy when you SPRAY at them from medium range (I know!) while scoped. It has virtually no recoil while scoped, and it'll usually get you out on top, unless they l9 you like a nub. Which brings me to another gun: the l9, if you're fast. I suppose, if you're REALLY lucky, you could use the M4A1 at medium range as well, because it miraculously likes to headshot from weird ranges at weird times. Like, no recoil, either. You can just spray and spray. But seriously, the FAMAS does that too, so yeah.
Long range: M24, by all means. Headshot machine. Get some practice at quick aiming for the head and people will freak out and call you an aimbotter. You can outsnipe L9 users from long range simply because their gun can't hit you while you're already headshotting the next person you see. ;D Uber smex. I love this gun. And if you tend to rush your snipe victims and generally miss because you're too eager for that next headshot, get the MSG-90 or SR25, because they're almost as accurate as the M24 and you can spray at your enemies with them. Hope I've helped. :O
