Total metal jacket (TMJ or full metal case) bullets are made by electroplating a thin jacket of

ductile Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...

metal (usually

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

) over a core of different metal requiring protection from abrasion or

corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...

. Similar

full metal jacket bullet A full 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 as gilding metal, cupronickel, or, less commonly, a steel alloy. A bullet jacket usu ...

s mechanically swage a thin sheet of metal over the core. The swaging process leaves an opening exposing the core on the base of the bullet, while electroplating deposits a jacket over the entire bullet surface. Protecting the base of a

lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...

-core bullet from burning powder gas may prevent molten lead from being released as a fine spray in turbulent gas leaving the muzzle of a firearm. Electrolytic deposition can create very thin jackets, while jackets applied by swaging must be thick enough to avoid being torn during the swaging process. Uniformly electroplated jackets may produce more accurate bullets than jacket and core materials deformed during swaging. Thinner electroplated jackets may not be as strong as conventional swaged jackets of similar metal, and may not prevent leading at higher bullet velocities. Thinner jackets may be damaged by and foul barrel ports for

gas-operated reloading Gas-operation is a system of operation used to provide energy to operate locked breech, autoloading firearms. In gas-operation, a portion of high-pressure gas from the cartridge being fired is used to power a mechanism to dispose of the spent ...

or recoil compensation.

Sources

Handloading Bullets {{Ammo-stub