A full metal jacket (FMJ) bullet is one which has a lead or other soft metal core, completely coated in a hard metal, such as copper, gilding metal, and sometimes a steel alloy. The jacket covers the entire part of the bullet visible from the case.
Although usually full metal jacket ammunition has a lead core, it is not always the case. The British Mark 7 .303 ammunition had the front third of the inner core made of a lighter material, like aluminum or wood pulp. This caused the bullet to yaw violently when it hit its target.