Well, here's some information that you might not know about MW2 and if you do, good for you.
There are two weapons in the game which have problems. The MG4, and the M9. You may not have noticed because you're most likely playing by yourself but both of their Silencers are duds. They don't work, you can shoot with the silencer on and you'll still show up on the map.
Also, another thing which is actually quite helpful is a glitch with Cold-Blooded. If an enemy helicopter is destroyed, but hasn't actually blown up and is just circling, you should shoot it with any weapon; if you have Cold-Blooded on. When you do, each bullet that hits counts as another killed killstreak with cold-blooded on.
http://www.youtube.com/watch?v=Ix98lGw4qK8&feature=PlayList&p=6DA5FBC1B6A1BFDD&playnext_from=PL&playnext=1&index=23