Are you on PS3 on 360? Its a lot easier if you team up with someone.
Btw in MW3 you do have times when it feels you shot 10 bullets into someone and they turn around and shoot 2 to finish you off. I have been hearing its to do with theater mode, having to record every second of your gameplay while you are playing might be slowing you down, so might as well turn it off.
Also I can say for sure its not the guns because you unlock some good guns really early. If you are having troubles, this class can help: PP9901 with rapid fire, range, sleight of hand, hardline, steady aim, just hip fire like a boss.
Also the scar-l is really good and is unlocked quite early. Later on you do get the acr which I find to be quite beast, as well as the G36. The best AR by far seems to be the Type 35 since it destroys with pretty much one burst.










