FMJ



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.

Call of Duty: Modern Warfare 2
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 all primary weapons.

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.

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 sides 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 takes numerous amounts of kills.

Trivia

 * FMJ and Rapid Fire are the only attachments that aren't visually different by any way other than in the pick-up text. They are also the only attachments that aren't featured in the campaign.
 * The MG4 has an incorrect pickup name and inventory name; it appears as "Explosive Rounds". The aesthetic properties of this attachment are more in line with this name.
 * 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.
 * In-game, FMJ bullet holes are round. Non-FMJ rounds make elliptical holes.
 * FMJ and the ACOG sight are the only attachments available for every weapon, excluding the Launchers.
 * FMJ removes the Grip on the M4A1, but it is still held as if a grip were still there.