<P> A full metal jacket (FMJ) bullet is a small - arms projectile consisting of a soft core (often lead) encased in a shell of harder metal, such as gilding metal, cupronickel, or less commonly a steel alloy . In military nomenclature, it is often labeled ball ammunition . </P> <P> The use of full metal jacketing in military ammunition came about in part because of the need for improved feeding characteristics in small arms using internal mechanical manipulation of the cartridge to chamber rounds as opposed to externally hand - reloading single - shot firearms . The harder gilding was less prone to deformation than softer exposed lead, which improved feeding . It is often thought that military use of FMJ ammunition was the result of The Hague Convention of 1899, Declaration III, prohibiting the use in international warfare of bullets that easily expand or flatten in the body, but this only addresses that projectiles must not be designed to flatten or expand, it only mentioned what is prohibited not the technical details for acceptable ammunition . Jacketed bullets had been in use since at least 1882, seventeen years prior to the Hague Convention . However, the original treaty making a requirement for Armies to use a bullet that did not fulminate, expand or flatten was the Declaration of Saint Petersburg of 1868, which preceded the development of the FMJ by twelve years . The round was invented by Swiss Col Eduard Rubin who was working for the Swiss Federal Ammunition Factory and Research Center which developed ammunition for the Swiss military . Since the Swiss were signatories to the 1868 Declaration; his invention of the FMJ would have been bound by the strictures of this treaty . </P>

What does the term full metal jacket mean
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