I just LOVE the M4 Carbine.. It's easily my Favorite Weapon that ive used.
At the advice of others, I tried the AK47 and the AK47-u, and both are great guns.. but for me, someone who doesnt really shoot from the hip, but isn't super-accurate, the M4-Carb is the best thing. It has a long range, quick firerate, and good accuracy. While its damage is quite low. I've found that it fires fast enough and accurate enough that it makes up for it. (I just had to get used to shooting a much LONGER amount of time then I think i should. Usually, the last bullet before I stop is the killing one.
Also, I find that the Grenade Launcher, while some may say its "Noobish", is a GREAT resource to have at your disposal.
Say for example, you shoot someone, and you know they are hurt, but are hiding around a corner. Just pull out your grenade launcher, and fire at the floor just beyond the corner.. and BAM, either their even more hurt, or (most often) dead!
It's also useful against someone you know is way more accurate then you are, yet you know exactly where they are.. Just poke around the corner, fire, and run and hide. Often times it will kill them. You just mainly have to be sure to be far enough away. And only SWAP to it, dont run around with it out. When you do that, you run the risk of running into someone, and not having a way to fight back (or one shot)
Anyway. thats what I do