FMJ

Full Metal Jacket, often shortened to FMJ, is a weapon attachment in Call of Duty: Modern Warfare 2. Its effects on penetration are completely identical to those of Deep Impact from previous Call of Duty games. It does not increase damage against exposed targets. Instead, it reduces the amount of damage lost when shooting through materials.

If a player fires a weapon with the FMJ attachment, the bullets will be red tracers and a small rain of sparks will shoot from the bullets' impact point. This makes them easily visible so others can tell 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 bullet impact.

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

Real-World Usage & Nomenclature
An FMJ bullet usually consists of a soft lead core with a relatively hard copper casing around it (hence the 'Metal Jacket'). Since there is a hard outer casing the bullet will tend to expand less than a soft or hollow pointed bullet, exerting its force on a smaller area and giving it more penetration. The rear face of a FMJ round is not covered by the jacket.

The jacket protects the core from deforming on certain types of material, like tree branches or window glass. By contrast, a non-jacketed round would receive significant deformation, and potentially fail to penetrate its target.

Many believe that someone hit by this type of round would be less likely to die on impact, as the bullet would create a narrower wound channel. However, many FMJ rounds are designed to tumble or fragment in order to increase their wounding potential. Early 5.56 rounds (55 grain) will tend to tumble, while the newer and heavier 5.56 rounds are designed to fragment.

Opposite the FMJ bullet is the JHP (Jacketed Hollow Point) or JSP (Jacketed Soft Point). Designed for expansion and rapid wounding without regard to armor penetration, they are commonly used in pistol and hunting rounds. A Total Metal Jacket encloses the core entirely in jacketing metal. Armor piercing ammunition is typically copper jacketed, with a core of either steel, tungsten carbide, or any other hard metal.

The Hague Convention of 1899, Declaration III, makes military use of "any expanding ammunition" (such as hollow point rounds) illegal by international law.

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 to begin with.
 * FMJ bullets appear to explode on impact, which is inaccurate as full metal jackets simply change the way that bullets go through objects. Armor-piercing rounds are known to shed their jacket upon penetrating hard material, but this would not cause bright explosions as it does in the game.
 * Some weapons like the M240 have the incorrect pickup name and inventory name; they appear as "X EXPLOSIVE ROUNDS." The aesthetic properties of this attachment are more in line with this name.
 * FMJ is the only attachment that is available on every gun, with the exception of launchers, which do not have attachments.
 * Extended Mags is arguably the most time-consuming attachment to achieve, as it requires the player to get 40 bullet penetration kills with FMJ as an attachment, ie. a complete fucking ball-ache
 * FMJs are good for urban environments as there are many walls that FMJs can more easily penetrate.
 * For any weapon, firing through an intact window or grated floor or even other players with FMJ counts as a penetration kill.
 * In-game, FMJ bullet holes are round. Non-FMJ rounds make ovular holes.
 * Players can equip FMJ rounds on shotguns, although no such thing exists; the closest comparison would either be flechette rounds or rifled slugs. Flechette rounds performed poorly in their intended role, due to their light weight and ballistic instability, and rifled slugs are not in and of themselves armor-piercing projectiles, but were merely intended to extend a shotgun's maximum effective range.
 * Contrary to popular belief, FMJ does not increase weapon damage, it just increases weapon penetration, serving as an equivalent to Deep Impact.