FMJ

Full Metal Jacket, often shortened to FMJ, is a weapon attachment in Call of Duty: Modern Warfare 2, it is the Perk Deep Impact's replacement, it increases damage done when penetrating surfaces such as walls. It does no extra damage when firing at a target out in the open, but rather is intended to help kill players behind cover.

If a player fires a weapon with the FMJ attachment, a small rain of sparks shoots from the bullets' impact point, making it easily visible for other players to notice whether or not an enemy player has the FMJ attachment. For some weapons, bullet holes appear larger while using FMJ. It also changes the sound of the weapon, primarily the sound of impact, but also makes some weapons louder. The easier way to acknowledge a FMJ user is the impact effect when the bullet hits something, it glows a bright yellow spark instead of discharging normal dust. It is normally used with bling and another attachment so the player can penetrate cover more easily and still have another attachment.

Getting 40 bullet penetration kills with the FMJ will unlock Extended Magazines for most weapons.

Real-World Usage & Nomenclature
An FMJ bullet usually consists of a soft core of a dense metal and a very hard outer casing around it (hence the 'Metal Jacket'). Since there is a hard outer casing the bullet does not expand so its force is exerted on a smaller area, giving it more penetration. Many believe this would mean that someone hit by this type of round would be less likely to die on impact (as the bullet would pass straight through them, so the full force of the bullet would not be transferred to the person). However, with some varmint-based rounds, this would not be the case, as the bullets weight is greater in the rear the bullet will hit the target and shift base forward, this is why the 5.56 round does passable damage, as it's high velocity causes it to "tumble", causing lethal wounds, but what many consider to be insufficient stopping power (which is not shown in the game). JHP (Jacketed Hollow Point) expand so all of the energy from High-powered rifle rounds, such as the 7.62X51mm of the SCAR, FAL and M14 EBR, and the .30-06 of the M1 Garand, have far more energy than these smaller rounds, and shots fired from such a weapon still possess more than enough energy to send an expanding bullet through a person. There would still be the added benefit of a larger wound with an expanding round, however as ammunition of this power typically turns the areas surrounding the wound into jelly (as the shock wave of the bullet tears the tissue apart) so the added benefit might not have much effect.

The Hague Convention of 1899, Declaration III, makes military use of "any expanding ammunition" illegal by international law. Therefore, the "FMJ" name for this weapon attachment is a bit misleading, as the "normal" ammunition would be FMJ as well. "Armor Piercing" would have been a more correct name. Armor piercing ammunition is typically copper jacketed, with a core of either steel, tungsten carbide, or any other hard metal. Regular FMJ is commonly copper jacketed with a lead core.

Trivia

 * The in-game version of FMJs is more likened to real-life Armor Piercing rounds, particularly since, as mentioned above, most real-life military forces only use FMJ rounds in small arms.
 * As seen in the game the bullets actually explode on impact which is false as full metal jackets just change the way the bullet go through an object. However, armor-piercing rounds are known to shed their jacket upon penetrating hard material.
 * Some weapons like the M240 have the incorrect pickup name and inventory name, as "X EXPLOSIVE ROUNDS". The properties of this attachment go more along the lines of this name.
 * FMJ is the only attachment that is available on every gun.
 * Extended Mags is arguably the time-consuming attachment to achieve, as it requires you to get 40 penetration kills with FMJ as an attachment.
 * FMJs are good for urban environments as there are many walls that FMJs can easily penetrate.
 * For any weapon, firing through an intact window with FMJ counts as a penetration kill.
 * In-game, FMJ bullet holes are round. Non-FMJ rounds appear ovular.
 * In-game players can equip FMJ rounds on a shotgun; in reality, shotguns do not fire rifle-type cartridges, the shotgun equivlent to FMJ rounds would be buck shot