FMJ

FMJ (Full Metal Jacket) is a weapon attachment in Call of Duty: Modern Warfare 2 and Call of Duty: Black Ops II.

Call of Duty: Modern Warfare 2
FMJ is the replacement to the perk Deep Impact from Call of Duty 4: Modern Warfare, allowing bullets to penetrate surfaces more easily.

Contrary to popular belief, the attachment does not add damage to the weapon using it against enemies in the open - though the in-game stats show an increase in damage. It allows the bullets to penetrate thicker surfaces, as well as losing less damage through other 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, while also changing the sound of the bullet impact. For some weapons, bullet holes appear larger while using FMJ. It is also worth noting that while it does not make the bullets themselves louder, the impact of an FMJ bullet makes very audible sounds that any player within ear-shot can hear, even if that player is using FMJ in conjunction with a silencer. Because of this, if the player hits a surface when targeting an enemy, the enemy might hear the shot and escape, or turn and shoot back.

In game, the FMJ is effective on multiplayer maps with lots of cover, such as Skidrow, the offices in Highrise, and the bunkers in Afghan.

FMJ on a Sniper Rifle 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 unlocks Extended Magazines for that primary weapon.

Shooting through windows is a highly effective way for a sniper to get FMJ bullet penetration kills for a sniper rifle (especially with the Intervention and Barrett .50cal, which have high damage multipliers). 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, intact 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.

In Sub Base, the two-story building with generators on the bottom floor and what appear to be servers on the top has a penetrable fence around the generators on the bottom floor. Note that only the north side of this fence can be effectively "penetrated" while shooting through the other side will have no effect.

FMJ can also help the player achieve the "X-Ray Vision" challenge easier, however, as the X-ray Vision III(final) requires only 15 penetration kills, most players get the challenge before unlocking FMJ, as for most weapons, FMJ requires 300 kills to unlock.

To achieve FMJ kills, a player could go to 3rd Person Team Tactical and wait behind corners and such, then shoot the enemy right through the cover.

It is quite easy to achieve FMJ kills in game modes like Domination, Demolition, Sabotage and their Hardcore versions. All the player must do is camp at one of the objectives and when the game notifies an enemy is capturing or planting the player may shoot through the objective or any close cover to achieve their FMJ kill.

Call of Duty: Modern Warfare 3
FMJ was planned to be used for Call of Duty ELITE, but was scrapped and remained unfinished, hinting that it was going to appear in Call of Duty: Modern Warfare 3 at some point. It seemingly was replaced with the Weapon Proficiency Impact.

Call of Duty: Black Ops II
The FMJ returns in Call of Duty: Black Ops II. In addition to greater penetration, damage is increased against scorestreaks, giving an effect similar to Blind Eye Pro from Call of Duty: Modern Warfare 3.

Trivia

 * The MG4 has an incorrect pickup name and inventory name when used with FMJ; it appears as "Explosive Rounds".
 * For any weapon, firing through a window, grated floor, and even other players with FMJ counts as a penetration kill if a bullet hole is seen in the surface afterward.
 * The player can unlock the FMJ emblem by completing Dictator challenge (fire an entire LMG magazine into an enemy without missing).
 * In-game, FMJ bullet holes are round. Non-FMJ rounds make elliptical holes.
 * FMJ is the attachment compatible with the most weapons, being compatible with every weapon except Launchers and the Riot Shield.
 * FMJ removes the Grip on the M4A1, but it is still held as if a grip were still there.
 * Sometimes, the particles that produces the FMJ bullets upon collision with a surface can slow down the game. This is more noticeable when using a weapon with a higher rate of fire, or when firing into a surface which the player is right up against.
 * It appears that when FMJ rounds hit a solid surface, they "explode" as red sparks will fly from the impact point.

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