Primers are devices that, when sharply struck, burn or explode to provide the heat source required to ignite the propellant charge. All modern small arms primers are chemical initiators.
- provide an initial spark or flame,
- establish the preignition pressure for the main charge,
- provide a gas seal for the cartridge.
For decades, mercury fulminate was the most commonly used primer. Although contemporary with fulminating powders containing potassium chlorate (whose residues promoted rust), mercuric priming was preferred because it did not rust the gunmetal.
When brass cartridges were adopted for use, it became apparent that mercuric residues (when in contact with brass), resulted in brittle cartridge cases. Such cartridges were fine for the first firing but could not be safely reloaded. The brittle brass could fail the next time it was fired, releasing hot gases. Thus, mercuric priming was replaced with chlorate priming. The U.S. military arsenals abandoned mercuric priming in 1898 at the beginning of the smokeless powder era.