Stick with it mate.
Some of those stock guns are actually very good too.
M16 (first assault class rifle, US only until later)
M4 (first engineer class rifle, US only until later)
Those are two of the best guns in the game, similarly the US starting gun for the Support class is also incredibly good, the M27.
It's more about unlocking a few attachments for each gun that let you use them how you want, for example i'm happy to use any of the semi-auto sniper rifles, but only once i've got a red dot or holographic sight unlocked for them so I can use them at closer ranges.
When you first unlock a gun though it has no attachments available, but 10 kills with that weapon later and you generally have something to use in place of iron sights (some guns have horrible iron sights).
Also worth playing the co-op as there are co-op specific unlock able guns, and actually these are pretty good, the KH2002 has always been one of my favourite assault rifles and it gives you a little practice at the game before getting too heavily into getting your ass handed to you in full multiplayer...also the second level lets you get to grips with the basics of piloting a helicopter, something I found invaluable and i'm actually a reasonably competent pilot now largely thanks to having the breathing space in co-op to learn.
Take it from me (someone who has unlocked every gun, every vehicle upgrade and a good percentage of the way through unlocking every attachment for every weapon) this game is very balanced with weapons, there's no one gun to rule them all, there's a lot of personal preference, for example some people think the Famas is a really good assault rifle, I think it's a load of crap because it takes longer to reload than it does to empty it's clip and it's got NO accuracy...however, at ranges less than a few yards it's basically unstoppable. I on the other hand think the M16, with it's reload speed a clear second quicker than any other gun in it's class, it's slower paced fire rate, deadly accuracy and ability to sustain fire for longer make it a much better choice as it's a superb all rounder with far less time between firing with it's lower reload times. All personal preference though (except people who think the famas is good are just wrong) and once you get the hang of it you should be able to just about pick up any weapon and be good with it provided you play to that weapons advantages.
