How should I get better at Call of Duty Modern Warfare 2?

I find that the TAR-21 is more effective than the ACR. I would recommend using Ranger with Akimbo and FMJ as your secondary, Stopping Power (to do even more damage), Bling (for FMJ and what sight you want to use) and Steady aim, so that both of your weapons are more accurate so that you can start shooting before you aim down your sight. This is the cl I have used and i have gotten 5 nukes with it, so it should prove effective for you.