A ‘Primer’ About Rimfire Vs. Centerfire Ammunition

by Jason J. Brown Note: This article was originally posted on NRA Blog

Ammo
Anyone familiar with firearms can tell you: there are loads of different kinds of ammunition.

USA -(Ammoland.com)- 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.

Rimfire cartridges. (Photo courtesy/Shooting Mystery)

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.

Comparison of rimfire and centerfire ammunition. (Graphic courtesy/Accurate Shooter)

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).

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.)

Centerfire cartridges. (Photo courtesy/The Hunting Gear Guy)

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.

(Illustration courtesy/SWG Gun)

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!

  • 5 thoughts on “A ‘Primer’ About Rimfire Vs. Centerfire Ammunition

    1. I have a Veterlin model 1869 bolt action rifle in 10.5mm Swiss caliber (0.45″). The ammo is rimfire. The rifle has two firing pins. There is only a limited amount of rimfire ammo available which is dated from the time when the rifle was in active service in some European armies. The conversion of the bolt to center fire is relatively simple for a competent gunsmith and, there is center fire brass available. I have not converted mine.

    2. So this came from NRAHQ. Perhaps they should get someone who has a basic knowledge of ammunition to proofread their articles. That might prevent them from printing this: “0.209-inch, used for shotgun shells and inline muzzleloaders”. Ignorance abounds.

        1. And if you weren’t such an ignorant person, and could read, you would know that that “209” refers to the type of battery cup as opposed to a “57” battery cup which is smaller. The article, which I quoted, stated “0.209-inch” which is just a fools statement. Over the years Remington switched to the larger 209 battery cup assembly. And, by the way it is possible to reload the 209 cup with large rifle or large pistol primers. But then you would know that if you weren’t so ignorant.

    3. ? Rimfire is unreliable? I’ve shot a lot of 22LR – more roinds than center fire – over the years and don’t remember reliability issues. Maybe it was the guns I used – a really old Savage, a Ruger Standard Mark II and a Ruger 10/22.

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