Anyone familiar with firearms can tell you: there are loads of different kinds of ammunition. Walk into most sporting goods or local gun shops and you’ll find a huge variety of ammo in different calibers, ranging from smallbore plinking rounds all the way up to hulking .50-caliber long-range ammo, and loads for pistols, rifles, shotguns, and more.
Despite the seemingly endless variety of rounds available, virtually all of them fall into two types of ammunition: rimfire and centerfire. These two kinds of ammo are named according to how the primer ignition system works.
Each round of ammunition, whether rimfire or centerfire, is referred to as a cartridge. Each common cartridge contains four parts – a case, primer, some form of propellant, and the bullet or projectile. The term “bullet” is sometimes mistakenly used in place of “cartridge,” but in reality only refers to the projectile, not the whole assembly.
When a gun is fired, the firing pin impacts the primer of the cartridge, creating a small explosion that ignites the propellant, inside the case. That ignition propels the bullet out of the case down the barrel, heading toward your target. The difference between rimfire and centerfire ammunition exists in the how the primer systems work.
Rimfire Ammunition As mentioned, rimfire ammunition is so named because the priming compound is spun inside the rim of the case. When the firing pin strikes the rim of the cartridge, it will ignite the primer. Rimfire cartridges are typically limited to lower pressures, leading to relatively small calibers, like the common .22 Long Rifle (LR).
Comparison of rimfire and centerfire ammunition. (Graphic courtesy/Accurate Shooter)
Rimfire ammo dates back to 1845, when French inventor Louis Nicolas Auguste Flobert conceived and created the first rimfire metallic cartridge. Born from an 1831 patent, Flobert’s cartridge featured only a percussion cap with a bullet affixed to it, but no powder. These cartridges were designed for use in indoor shooting galleries, and produced low velocities.
The .22 Short, developed for the Smith & Wesson Model 1 revolver, was introduced in 1857, using black powder to propel a conical projectile. This began an evolution of rimfire cartridges resulting in .22 LR, which became one of the most popular cartridges in history.
While the .22LR is the reigning king of rimfire calibers (with company from newer kid on the block .17 HMR -- Hornady Magnum Rimfire), early rimfire rounds included bigger calibers, including .30, .32, .38, .41, .44, and even the large .58 Miller. The introduction of smokeless powder led to the abandonment of larger calibers, as smokeless powder generates much higher pressures than black powder. This resulted in a need to use less propellant, which demanded a scaled-down round. Firearms and ammunition makers found centerfire ammunition to be a much better choice for larger projectiles propelled by smokeless powder.
.22LR and .17 HMR rimfire cartridges. (Photo courtesy/GunsAmerica)
Centerfire Ammunition Centerfire cartridges host their primer in the center of the cartridge case head – hence the name. Today, the vast majority of ammunition produced is centerfire, with only smaller calibers being produced in rimfire, including pistol, rifle and shotgun ammo.
The primer in centerfire rounds is a metal cup that holds a primary explosive. The firing pin of the gun impacts the primer and crushes the explosive between the cup and an anvil, producing gas and incandescent particles to ignite the smokeless powder. There are two kinds of primers available for centerfire ammunition: Berdan and Boxer, both named for their respective inventors (and interestingly, both colonels.)
Berdan primers, invented by Col. Hiram Berdan of New York, patented the first version of his primer in 1866. Initially built to use copper shells, testing revealed problems with reliability, namely the swelling of the shell that prevented the cartridge from seating in the chamber. Berdan improved the design by switching to brass cases and adjusting how the primer cap was installed, creating a design that remains unchanged today.
Boxer primers, created by Col. Edward Boxer of the Royal Arsenal in Woolwich, England, received an English patent in 1866 and a U.S. patent in 1869 for his primer. Similar to the berdan primer, the primary difference is the location of the anvil. The separate stirrup design of the boxer primer makes no difference in performance, but allows for much easier removal of spent primers. This makes reloading the case much easier, as well.
Comparison of Berdan and Boxer centerfire primer systems. (Graphic courtesy/WeaponsMan)
As a result, all U.S. factory-made commercial centerfire ammo is Boxer primed, while a large amount of imported European ammo is Berdan primed, which is very difficult, expensive and time-consuming to reload.
Boxer primers come in various sizes – 0.175-inch, used for small pistol and small rifle rounds, 0.209-inch, used for shotgun shells and inline muzzleloaders, 0.210-inch, used for large rile and large pistol rounds, and 0.315-inch, used for .50 BMG and similar ammo.
Of note when choosing ammo, particularly in regard to your firearm cleaning regimen, a lot of military surplus ammo employs corrosive Berdan primers (by contrast, nearly all Boxer primers are non-corrosive.) These Berdan primers tend to be more reliable in austere environments, but can be hard for firing pins to strike accurately, and will leave corrosive materials in the barrel and action of the firearm after firing.
When using military surplus or ammo with corrosive primers, be sure to thoroughly clean your firearm after use to avoid pitting and damage to internal components, and consider installing an enhanced firing pin capable of properly impacting the primer to ensure proper ignition.
Want to learn more about the differences between rimfire and centerfire ammunition? Check out this episode of NRA Firearm Science with competitive shooting champion Jessie Duff!