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, although it does show a damage modifier when adding FMJ as an attachment. Instead, it allows the bullets to penetrate thicker surfaces and 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.

In game the FMJ can be quite useful on multiplayer maps with lots of cover such as parts of Skidrow or the offices in Highrise and for assaulting the bunkers in Afghan as you can shoot through walls.

It is generally inadvisable to use FMJ on a Sniper Rifle as it causes the bullet animation to appear to "spread", making it much easier to see where shots fired from a FMJ-equipped sniper rifle come from.

Getting 40 bullet penetration kills with the FMJ will unlock Extended Magazines for that primary weapon.

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; This depends a lot on the use of the rounds. 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. By comparison, pistol rounds, which contain significantly less energy than rifle rounds, rely more on the expansion from the hollow-point to create a large wound cavity. A Total Metal Jacket encloses the core entirely in jacketing metal.

Armor piercing ammunition typically contains a solid steel core such as the original 7.62x54R Mosin Nagant rounds. There are also High Explosive/Incendiary rounds, which means that there is a central mass of tungsten with a cartridge of high explosive and an incendiary tip, within a copper jacket. It's designed so that when it impacts a surface, the copper jacket deforms, igniting the the high explosive and launching the tungsten penetrator forwards through the weakened armor. This type of munition is sometimes used with the Barrett M82, since this allows the engagement of targets inside hardened vehicles. This round can also be used to destroy the engine of a moving car or truck with a single shot. Other less effective variants are 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 is one of two attachments that do not appear in campaign, the other being rapid fire.
 * 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.
 * The MG4 and M240 have an incorrect pickup name and inventory name; they appear as "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.
 * FMJs are good for urban environments as there are many walls that FMJs can more easily penetrate.
 * For any weapon, firing through an intact (even partially broken counts) 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, steel shot, 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.
 * The FMJ emblem can be unlocked by completing the challenge, "Dictator" (fire an entire LMG magazine into your opponents without missing).
 * Shooting through windows is a highly effective way for a sniper to get FMJ bullet penetration kills for his sniper rifle (especially the two higher damage rifles, the Intervention and Barrett .50cal). If a sniper can get a one-shot kill (Stopping Power makes this easier) with their first shot through the window, it will only crack. The sniper can then take a second shot through the window (it may help to find a decent-size, uncracked piece of the window through which to view a high-traffic area) and, hopefully, get a second one-shot kill, making each window potentially worth two FMJ bullet penetration kills.
 * Oddly, if FMJ is attached to the M4A1 with no other attachments, the foregrip dissapears, though the character still holds it as if it were there.