A full metal jacket (or FMJ) is a bullet consisting of a soft core (usually made of lead) encased in a shell of harder metal, such as gilding metal, cupronickel or less commonly a steel alloy. This shell can extend around all of the bullet, or often just the front and sides with the rear left as exposed lead. (A bullet that is completely enclosed by the shell is alternatively termed a total metal jacket round.) The jacket allows for higher muzzle velocities than bare lead without depositing significant amounts of metal in the bore. It also prevents damage to bores from steel or armor-piercing core materials. The appearance of FMJ ammunition is highly distinctive when compared to hollow-point or soft point bullets. Historically, the first successful full metal jacket rifle bullets were invented by captain Eduard Rubin of the Swiss Army in 1882.
Full metal jacketed bullets have different properties, both in terms of behavior inside the barrel of the gun and also in flight. Whereas hollow point and soft-tipped bullets are designed to expand upon impact, fully metal jacketed bullets are technically limited in mechanisms to increase round expansion. In some cases this leads to smaller target damage, although not in all instances. For example, the 5.56x45 NATO round used in the M16/M4 family of firearms tends to spin vertically upon impact, leading to a massive cavity.
FMJ with variable coresEdit
Although British Mark 7 .303 ammunition is compliant with the terms of the Hague Convention, it creates more destructive gunshot wounds than standard spitzer bullets due to its internal design. The centre of gravity of the Mark 7 bullet is deliberately shifted towards the rear. This is achieved by constructing the front third of the interior of the bullet from a lighter material such as aluminium or wood pulp. The result is a tail-heavy FMJ bullet which yaws violently after hitting the target.