Full metal jacket bulletbullet that is encased in a copper-coated steel or gilding metal jacket, designed to stop the bullet from fragmenting within its target. The jacket prevents deformation of the bullet in the barrel or feed mechanism, from dirt overpressures or damage outside the gun. This reduces misfires. The jacket also prevents fragmentation, and the coating helps prevent damage to the gun barrel.
FMJ ammunition was introduced for ostensibly humanitarian reasons, as the Hague Convention of 1899 prohibits the use of expanding or fragmenting bullets in warfare. (It is commonly but incorrectly stated that this prohibition is in the Geneva Conventions.)
They have the advantage in warfare that they often injure their target rather than kill outright, creating a casualty that needs to be cared for, rather than a corpse. In this way, FMJ bullets can be more effective at consuming an enemy's resources than fragmenting bullets, yet the outcome of the victim is usually the same, death (or at least long-term removal from the field of battle, which is close to equivalent in military terms). Furthermore, because the bullet does not expand, FMJ bullets are much more effective at armor-piercing than hollow point bullets.
- Declaration (IV, 3) concerning Expanding Bullets, made at the 1899 International Peace Conference at The Hague, which entered into force on September 4, 1900