All About Primers - Inside MDT

Posted by Al Voth on 2024 Feb 22nd

All About Primers - Inside MDT

Every modern metallic cartridge has four components; the projectile, cartridge case, propellant powder, and primer. Of all those, I suspect the least understood is the primer, the little jewel that starts the entire ignition process of the cartridge.

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Modern primers need three basic components to function: the impact-sensitive priming compound, a container for the compound, and an anvil. However, not all of these components are necessarily contained within the primer. The simplest modern primer is found in rimfire cartridges. Here, it merely consists of a priming compound spun into the hollow rim of the cartridge case. When the firing pin strikes the rim, this impact-sensitive compound is crushed, sending flame into the cartridge's propellant powder, igniting it and thus launching the bullet. This makes it appear that a rimfire primer is a single component, but that's misleading. The cartridge's rim is the container for the compound, and the chamber of that rimfire firearm has a portion of it serving as an anvil, providing a solid surface on which the firing pin can impact and perform its crushing function.

The priming compound inside a rimfire 17 HMR cartridge.

Centerfire primers found in modern rifle ammunition are self-contained in a metal cup that serves as the container to hold the priming compound and an anvil.


Primer sizes are a significant issue for hand loaders, and for most of us, it boils down to two choices, small or large. If you shoot the big cartridges, like the .50 BMG, you can add that as a third size. According to industry standards, small rifle and small pistol primers are the same physical size, However, large pistol and large rifle primers are sized slightly differently, with rifle primers slightly larger in diameter. They are also somewhat taller. Of course, their primer pockets are sized accordingly.

Also, primers are available in varying "strengths," largely based on the amount of powder required to ignite and the resultant pressure they are expected to contain. Therefore, pistol primers are designed to contain only pistol-level pressures and ignite pistol-quantity amounts of powder. Both rifle and pistol primers can also be had in "magnum" configurations, a variation designed to efficiently ignite the larger powder volumes in magnum cartridges. Rifle and pistol primers should never be substituted.

Stacking large pistol and large rifle primers side by side shows the size difference.


Most of the magic in a primer is located in the small amount of impact-sensitive chemical compound, which ignites when struck by the firing pin. The most common composition currently in use comprises approximately 40% lead-styphnate acting as the explosive, 40% barium-nitrate as the oxidizer, 10% antimony-sulfide as fuel, and 10% miscellaneous materials. Note the presence of heavy metals, especially lead. That lead in the primer contributes significantly to the airborne lead generated by a gunshot, and since lead is a hazardous material, manufacturers are working hard to get rid of it.

The three components of modern centerfire primers are a cup-shaped container, an impact-sensitive compound, and an anvil.

While lead-free primers have been available for several decades, they aren't universal. For now, most manufacturers are clinging to the old formulae. One exception is training ammo intended to be used in indoor ranges. After all, primer failures in training aren't a big deal, while keeping range air clean is.

A notable exception comes from Federal Ammunition. They have developed a lead-free primer they are calling the Catalyst™. This primer appeared first in some of their handgun training ammunition, and the formulation has recently migrated to their Black Cloud™ shotgun ammo. That step is a big deal, as lead-free primers in hunting or match ammo are a real rarity. I'm led to believe that as the product proves itself, it will also be integrated into other product lines. If this formulation works in the real world, it'll be a good move toward eliminating lead contamination, with a nice side benefit of cleaner-fired cartridge cases and guns.


Obviously, handloaders need to make careful choices about primers. There are some considerations besides getting the right physical size and a primer designed for either rifle or handgun. A handload that is safe and accurate with one primer won't necessarily be either if the primer is switched to another brand. So, never switch primer brands without reducing powder charges and working up loads carefully from there.

Primer hardness is another factor to consider, and it refers to how much energy is required from the firing pin to ignite the primer. Some primers require less of a blow than others to start the firing sequence. However, determining relative primer hardness is more difficult.

In my experience, the easiest primers to ignite are those made by Federal, while the hardest tend to be CCI brand. The other manufacturers fall somewhere in between; however, I can't comment on the offshore brands as I don't have enough experience with them. The hardest primers of all are those made for the military. This is largely because of the rough handling these cartridges get in full-auto firearms and the potential for an inertia-type firing pin strike when a cartridge is chambered but not fired. These inertia strikes show up as light firing pin dents on the primer because the firing pin slams forward when the bolt assembly suddenly stops as a cartridge is chambered.

Firing pin primer strikes by three different semi-auto rifles simply from chambering.

If you shoot a semi-auto rifle, it's educational to pull the bolt/slide to the rear and hold it there. Then, put a primed cartridge case (don't use live ammo!) in the chamber and release the bolt/slide from its rearmost position, allowing it to slam forward hard. Extract the cartridge case and see if there's a slight firing pin mark on the primer. If there is, primer hardness is one of the factors which keeps the cartridge from discharging. That should give you pause as to where the muzzle is pointed the next time you drop the bolt in a semi-auto to chamber a live round.

Sometimes, even people who shoot factory ammo need to worry about primer choices, with the cheap and popular SKS rifle being a great example. My factory-original 1950 Russian SKS will discharge commercial Winchester ammo in the slam fire test detailed above. Even feeding those cartridges from the magazine, which slows bolt velocity significantly, leaves a deeper mark than I'd like. But military ammo is fine because of the much harder primer used in that ammunition.

So, there's more to primer engineering than meets the eye. They may be the smallest component in a round of ammunition, but they're an important link in ammo performance and safety. That makes it worth learning about primers.



Al Voth calls himself a "student of the gun." Retired from a 35-year career in law enforcement, including nine years on an Emergency Response Team, he now works as an editor, freelance writer, and photographer and keeps active as a consultant in the field he most recently left behind—forensic firearm examination. He is a court-qualified expert in that forensic discipline, having worked in that capacity in three countries. These days, when he's not working, you'll likely find him hunting varmints and predators (the 4-legged variety).


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