Call Of Duty : Modern Warfare 2
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 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.
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 all primary weapons.