Tar-21 is the best assault rifle in MW2...LOL! I can't believe you have never used it. Put a silencer on it and you will become a God. Not to be so melodramatic but for real that gun is by far my favorite and go to gun in MW2.
It's a fully auto AR and has a pretty good iron sight. You can also shoot in burst for long range kills. You will love it.
Just watch my gameplay to get an idea