Afull metal jacket (FMJ)bullet is a small-arms projectile consisting of a soft core (often lead) encased in an outer shell ("jacket") of harder metal, such asgilding metal,cupronickel, or, less commonly, a steel alloy. A bullet jacket usually allows highermuzzle velocities than a lead alloycast bullet without depositing significant amounts of metal in thebore. It also prevents damage to bores from hard steel orarmor-piercing core materials.
Despite a widespread belief that the full metal jacket bullet was invented ca. 1882 by Swiss ColonelEduard Rubin while he was working for the Swiss Federal Ammunition Factory and Research Center,[1][2][3][4] in fact it was known already in the 1880s that the actual inventor was Prussian Major (later Lt.-Col.)Julius Emil Bode [es] (1835–1885), who came up with the idea in either 1875 or 1876.[5][6][7][8][9]
The use of full metal jacketing in military ammunition came about in part because of the need for improved feeding characteristics in small arms that used internal mechanical manipulation of the cartridge in order to chamber rounds, as opposed to externally hand-reloading single-shot firearms. The harder metal used in bullet jackets was less prone to deformation than softer, exposed lead, which improved feeding. Full metal jacketing also allowed bullets to withstand much higher velocities caused by the decrease of the caliber.[10]
In addition to the various advantages afforded by FMJ rounds, theHague Convention of 1899 Declaration III prohibits in international warfare the use of bullets that easily expand or flatten in the body.
By design, a fully jacketed projectile has less capacity to expand after contact with the target than ahollow-point projectile or asoft-point projectile. Although that can be an advantage when engaging targets behind cover, it can also be a disadvantage because an FMJ bullet may pierce completely through a target, leading to less-severe wounding and, possibly, failure to disable the target. Furthermore, a projectile that goes completely through a target can cause unintentional damage behind the target.[11]