How to get bullet penetration kills in MW2?

Attach FMJ to the weapon you are trying to complete the challenge with. Then follow an enemy with your fire as they run for cover. Or otherwise try to get them behind cover once they are hiding. Often, you can easily spot someone behind a window or a wall when the barrel of their weapon is sticking out. Easiest to shoot through is sheet metal like the most walls on Scrapyard.

An extremely easy and fast way to get Bullet Penetration kills is to play 3rd person team tactical.

When you aim down your sights, you look over your right shoulder, so if you are on the left side of a door and you aim down your sights, you can see down the hallway and not be seen by anyone in that hallway. Let your guns go and fire through the wall for the easiest Bullet Penetration kills.