Posted by XIII.
There is currently a "bug" in MW2 (I put it in quotes because it isn't so much a bug as it is overpowered weapons) that will allow you, if done right, to get 50+ kill streaks easily.
In order to do it, you must use the following:
Main gun: Any assault rifle
Attachment: Grenade launcher
Secondary: One Man Army Pro
Grenade: Claymore
Special: Your choice
Perk One: One Man Army Pro
Perk Two: Danger Close Pro
Perk Three: Your Choice
Death Streak: Your Choice (won't be needed, if done right)
Killstreaks:
5- Predator Missle
7- Harrier
11- Chopper Gunner
What you do:
Start up with your assault rifle and get the noobtube out. Aim and fire at the enemy. Repeat. Place your claymores down. Go back to safety and use one man army back to the class you were (your grenades will reload). Repeat. When you get the predator, it will do twice as much damage (thanks to danger close pro). You will most likely get your harrier now. Use it and it will easily get to 11 kills (thanks to danger close pro). Now use chopper gunner and watch how quickly you can kill everything. The bullets are twice as strong. Now either continue with your killing or kill yourself and start over.
By doing this, you can get 100+ kills with minimal deaths.
Here is a video of it in action:
Modern Warfare 2 Custom Kit Exploit - 100+ Kills
Also, if you want to be super annoying (although it is a tiny bit more difficult), swap predator missile for tactical nuke.
100+ Kill "bug" 
Linear Mode
