A full metal jacket bullet is a small-arms projectile consisting of a soft core encased in an outer shell of harder metal, such as gilding metal, cupronickel, or, less commonly, a steel alloy. Full metal jacket bullet · Contents · Impact characteristics · Cartridges with full metal jacket bullets. The term full metal jacket means the lead bullet is encased in a separate harder metal – this is typically copper, but can sometimes be other materials and even. This is a short (Ammo 101) demonstration on how FMJ is different from HP. Ammo For Beginners: FMJ VS Hollowpoint. Full Metal Jacket ammo, or FMJ for short, is a bullet style that uses a soft core, commonly lead, surrounded by a casing of harder metal.