How to get bullet penetration kills in MW2?

What do you have to do to get a bullet penetration kill in MW2? I've been stuck trying to complete the extended mags challenges because I'm not getting the penetration kills required.

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.

An easy way to obtain FMJ kills is by shooting through glass at your enemies. Some good levels for FMJ kills are Highrise, Terminal, Trailer Park (new map), Scrapyard.

You have to shoot and kill people that are behind something, like a thin wall.

Also make sure you have the FMJ attachment active on the weapon you are using, or it won't count as a kill in the challenge.

Just what the name implies really. It's counted as a bullet penetration kill when you kill someone who's behind a wall (even shooting through a railing can count), all while using the FMJ attachment.

It's probably the trickiest challenge to complete with any gun (certainly with sniper rifles).

Tip: When you see someone who's trying to run for cover, you've got a choice whether to for the chase or to shoot. In this case, shoot them through the wall. Doing this however will give your position away to your enemy.