Full Metal Jacket. Designation for a bullet which has a metal jacket (usually copper gilding, but sometimes steel, brass, or aluminum) surrounding the core (usually lead, but sometimes steel, tungsen, aluminum, or plastic). The bullet is designed to remain intact without deformation or expansion when striking a target. Most FMJ bullets actually have a base with exposed lead, but this is not visible when the bullet is seated in a cartridge case (see TMJ). The solid nose of an FMJ enhances feed reliability. FMJ ammunition is used by the military and is popular among private citizens for practice and plinking.