The self-contained metallic cartridge is one the most significant innovations in the history of firearms. It consists of four items—brass, bullet, powder, and primer. Any recent graduate of a hunter-ed program should be able to rattle off this information because illustrations showing the component pieces of cartridges have been a part of the curriculum for decades. What our bright-eyed new hunter might not know is that all four have a profound effect on cartridge performance, whether we’re talking about accuracy, reliability, or lethality. And not to pick on the poor kid, but he or she probably doesn’t have the slightest clue as to what a primer actually does. In this, our young hunter isn’t alone. Of the four building blocks of a cartridge, the primer is the least understood by a long shot. On top of this, a major advance in primer technology promises to make today’s good ammo even better.
Even the nerdiest reloaders—who can make a layperson’s eyes glaze over as they expound ad nauseum about the intricacies of the construction of a favorite bullet or their regimen for preparing brass to exacting standards—typically know little about the primers they fashion their ammunition with, other than that changing from one type to another might alter the sizes of their groups.
Asked to explain what a primer does, shooters often describe it as akin to a match that lights the powder at the tail end of the cartridge, initiating the process that sends the bullet along its merry way, or to the spark of a spark plug in the cylinder of a motor that ignites the compressed mixture of fuel and air.
Neither analogy is correct. The function of the primer is more complex than either a match lighting a fuse or a spark plug firing.
To understand what primers do, we have to delve into the chemistry that goes into the making of the volatile slurry that is pressed between the primer’s cup and anvil. That mixture, known as primer compound, is the most dangerous substance in the world of firearms and ammunition manufacturing. Though gunpowder is flammable, and the chemicals used to blue steel barrels are nothing you’d want to gargle with, mishandled primer compound will leave nothing but a smoking crater behind. There’s a reason that each individual primer gets its own protective cutout in the box it is shipped in.
The specific ingredients in primer compounds vary from one make to another. But the general formulas have remained remarkably stable over the decades, mostly because ammo makers—and, more to the point, their customers—are stubbornly reluctant to tinker with any recipe that deviates from the tried and true.
Above all, primers need to be consistent. If they don’t pop off when they’re hit by a firing pin—no matter how cold the temperature or how dirty and greasy the ignition system—shooters become exceptionally cranky and unforgiving. Igniting the powder charge, even at the cost of other desirable outcomes like accuracy, is the primer’s prime directive.
Despite this conservative bent, primer technology is experiencing a revolution right now. While there’s been plenty of hoopla recently about the newest high-performance bullets, powders, and precision brass, the fourth member of the cartridge quartet has undergone some interesting developments too.
By John B. Snow
