A full metal jacket (FMJ) bullet is a small-arms projectile consisting of a soft core encased in an outer shell ("jacket") of harder metal, such as gilding metal, cupronickel, or, less commonly, a steel alloy. A bullet jacket generally 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. In military nomenclature, it is often labeled ball ammunition.
