FMJ

Full Metal Jacket, often shortened to FMJ, is a weapon attachment in Call of Duty: Modern Warfare 2 that works similarly to Call of Duty 4: Modern Warfare ' s perk Deep Impact by increasing 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. It also changes the sound of the weapon, primarily the sound of impact. 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

 * FMJ can be used as an "attachment" on Shotguns. This is very incorrct, though, as a shotgun shell is made of plastic, not a metallic jacketing.  A correct name for an enhanced shotgun shell would be a "buckshot" or "magnum-grade".