Mercury fulminate is highly sensitive to heat and friction, which is what causes it to detonate on impact with objects. Throughout the 19th and mid-20th centuries, it was used as the explosive component of black-powder firearm caps as well as rifle and pistol cartridges and shotgun shell primers. The primer is the component of a cartridge that is responsible for initiating the combustion of the primary explosive charge (the gunpowder) in the cartridge. When the primer is struck by a firearm's hammer, the friction caused by the impact causes the mercury fulminate to undergo a exothermic reaction (detonation), and the combustion then, in turn, ignites the gunpowder. Mercury fulminate was replaced as the main component of primers in the late 20th century because it tended to degrade over time; it would break down into its constituent elements, and the mercury in it would react with the brass in the cartridge, weakening its structural integrity. It was replaced with less toxic and more stable chemicals like lead azide, lead styphnate, and tetrazene.
Mercury fulminate is also used in military-grade, low-explosive tipped ammunition; when the bullet impacts the target, it causes the bullet to explode, doing massive damage. Explosive ammo is typically used to better penetrate body armor; high-explosive ammo uses more volatile substances, like nitroglycerin. Explosive and armor piercing ammunition is regulated under Title II of the Gun Control Act of 1968 (GCA), which considers them destructive devices. This kind of ammo is generally illegal for civilians to own or make, with the exception of civilians with a specific type of Federal Firearms License (FFL). In order to sell firearms (machine guns, short barreled rifles, silencers) regulated by the National Firearms Act of 1934 (NFA) or destructive devices regulated under Title II of the GCA, a person with an FFL must also have a Special Occupational Taxpayer status (SOT). SOT has three different classes and is only granted to FFL dealers and manufacturers that pass an extremely thorough background check, who pay the extra tax fee, and own a legitimate licensed business that deals or manufactures firearms. A class 3 SOT costs $500 per year and allows a FFL to legally deal in NFA firearms & destructive devices, a class 2 SOT costs $1,000 per year and allows a FFL to manufacture NFA firearms & destructive devices, and a class 1 SOT costs $2,000 per year and allows an FFL to legally import these firearms and devices for other countries. Explosive and armor piercing ammo can only be sold by a Type 09 - Dealer in Destructive Devices, Class 3 SOT FFL, and it can only be made with a Type 10 - Manufacturer of Destructive Devices, Ammunition for Destructive Devices or Armor Piercing Ammunition, Class 2 SOT FFL.
