A Primer on Primers

Behold the primer—like Rodney Dangerfield, the simple primer does not get the respect it deserves, but the primer is the most critical component of metallic cartridges or shotshells, for it is the primer that ignites the powder charge that sends the bullet or pellets toward their targets. Before discussing the modern primer it is necessary to distinguish between explosives and gunpowders because many people, including many shooters, conflate the two. Explosives such as dynamite or nitroglycerine detonate when set off by a primer, violently and instantly (in as little as 1/25,000 of a second) converting the potential energy of their chemical compounds into kinetic energy that generate tremendous volumes of expanding gases which produce destructive shattering effects.

Exactly who invented the mechanical mixture of 75 parts of saltpeter (potassium nitrate), 10 parts of sulfur and 15 parts of charcoal that became black powder is unknown. The earliest known written formula for black powder was published in 1044 CE during the Song dynasty of China, but knowledge spread quickly. Black powder and modern smokeless powders generically known as gunpowders, burn at very controlled, though very rapid rates when ignited by a primer, generating gases that expand equally and with increasing pressures in all directions of whatever contains the gunpowder. Caveat: if the container is completely closed with no outlet for the expanding gases an explosion will occur. However, when the “containers” are cartridges or shotshells there is something that provides an outlet for the expanding gases—the bullet or pellets. The bullet or pellets are aligned with the barrels of rifles or shotguns and the rapidly increasing volume of gases propel the bullet or pellets out of their containers and down the barrels. Gunpowders are more properly termed propellants or propellant powders.

Various methods of igniting black powder progressed from a hot metal rod, then slow match (a length of cord impregnated with saltpeter)—the matchlock), flint and steel (the flintlock), the external percussion cap, to today’s modern metallic primer. The modern Boxer metallic primer consists of three components, the cap—a cup-shaped stamping made from rolled copper-alloy, the anvil—sometimes called a disc or stirrup piece that is placed inverted in the cup, and a pressure-sensitive priming compound. And yes, the priming compound is an explosive. In “clean room” conditions a small amount of priming compound, in a wet form, is pressed or rubbed into the cap and allowed to dry. In technical terms, when a firearm’s firing pin strikes the primer cap of a chambered cartridge or shotshell, the priming compound is crushed between the cap and the anvil, causing the priming compound to explode, and sending a stream of hot gases as a jet of flame and through the flash hole and into the cartridge case or shotshell. This flame ignites the propellant powder. There are many formulas for priming compounds and these compounds are adjusted to provide the proper brisance of flame for the propellant type and cartridge. Thus, there are “standard” and “magnum” primers, as well as “large rifle” and “small rifle” and pistol primers.

The earliest priming compounds contained fulminate of mercury, which was superseded by potassium chlorate, and when the cartridge was fired the mixture of burning gases produced potassium chloride as a byproduct—a compound very similar to sodium chloride—common table salt, and salt, being hygroscopic, will draw moisture from a humid atmosphere—and corrode metal! For many years “corrosive” primers were used for military and commercial metallic ammunition in the United States as well as other countries. Shooters were taught to clean their firearms carefully after firing “corrosive ammunition”, generally via the use of hot, soapy water, following with clean hot water to remove the salt and dry the interior of the barrels—the same procedure their shooting ancestors had used to clean the barrels of firearms using black powder.

Prior to 1955 corrosive primers were used for military ammunition manufactured in the United States (the one exception being the .30 caliber carbine ammunition) and most foreign countries. Beginning in 1955 ammunition manufactured for the American military was manufactured with non-corrosive primers. American commercial ammunition manufacturers switched from corrosive primers to non-corrosive primers following the end of World War II (there was practically no commercial ammunition manufactured during World War II since the commercial manufacturers’ facilities were contracted exclusively to manufacture ammunition for the American military). Other major ammunition manufacturing countries—such as China and Russia—continued to manufacture ammunition with corrosive primers through at least the end of the Twentieth Century. Large amounts of ammunition manufactured in China and Russia and marketed as “mildly corrosive”, particularly in the popular 7.62x39mm and 9mm calibers were sold in the United States. However, there is no such as “mildly corrosive” primers—primers are either corrosive or non-corrosive, as many American shooters discovered to their dismay when the interiors of their rifle barrels were eroded by rust.

There are two types of metallic cartridge primers, the Boxer mentioned above, and the Berdan, with the Boxer primer being used exclusively by American as well as most European commercial ammunition manufacturers. Berdan primers are used by Chinese and Russian manufacturers, and Berdan primers may be contractually specified for ammunition manufactured to military standards by various European nations. Interestingly, the Boxer primer was developed by Colonel E. M. Boxer, a British officer, and the Berdan primer was developed by Hiram Berdan, an American military officer. Both types of primers were patented in 1866, and while they are difficult to distinguish externally in manufactured ammunition they differ substantially internally. Berdan-primed ammunition has two flash-holes in the primer pocket with a small “bump” between them. The Berdan primer cup contains only the pressure-sensitive priming compound since the small “bump” acts as the anvil. Boxer-primed ammunition has a single flash-hole. Berdan primers are frequently mandated for military ammunition manufactured in countries other than the United States on the theory that ignition of the propellant powder via two flash-holes is more reliable.

As a practical matter, although Berdan-primed cartridge cases can be reloaded, the process of removing the discharged primers is complicated, and fired cartridge cases are generally discarded. A shooter desiring to reload cartridge cases after they have been fired should ascertain if the cartridges have Boxer primers rather than Berdan primers.

Much is demanded of the simple primer: it must be able to resist rough handling and shock, remain stable over a wide range of temperatures, endure long-term storage, and ignite with 100 percent reliability when chambered and struck by a firing pin. Once shooters realize the importance of primers, they will give primers the respect they so well deserve.