What is a Primer Made of?

A primer is a small, explosive metal cup inserted into the rear of a cartridge. Its purpose is to ignite the powder charge that sends the projectile (bullet or shot) flying through the air toward its target.

Understanding what a primer is made of and how it works is an integral part of learning how to reload ammunition.

How Primers Work

A primer is made of a small amount of reactive explosive that is set off by the impact of a hammer or firing pin. The mechanical strike of the firing pin causes the primer to ignite, sending a small flame front and particulate matter into the cartridge, igniting the primer powder charge.

This ignition source is needed by the stable gunpowder in a cartridge, which then begins its own chemical reaction that transforms it into rapidly expanding gasses that create the necessary pressure to push a bullet through the barrel of your firearm.

In many ways, the primer in your pistol cartridge works on the same principle used in demolition and mining operations where a smaller explosion from a detonator is used to begin a larger, more powerful chain reaction.

Modern manufacturers use two different designs for incorporating primers into their ammunition:

  1. Rimfire Primers – the primer is integrated with the rim of the cartridge and is not a separate component.
  2. Centerfire Primers – the primer is inserted into the brass casing and is removable. This type of ammunition can be reloaded.

Of these two, the centerfire design is far more common, which is good news for reloaders.

The 4 different primer sizes are typically broken down as:

  1. Small rifle primers – typically .175 inches in diameter and .120 inches tall.
  2. Small pistol primers – designed specifically for handguns, these are approximately the same size as small rifle primers.
  3. Large rifle primers – these measure approximately .128 inches in height and .212 inches in width.
  4. Large pistol primers – these are around .120 inches tall and .212 inches wide.

What is a Primer Made of?

Primers start with a thin sheet of copper alloy stamped into the appropriate-sized cups to fit your cartridges. A proprietary mixture of a shock-reactive explosive like lead styphnate, an oxidizer, and a fuel source will be added while it’s still in a less-reactive wet state.

Another sheet of metal will be stamped to create covers for these cups called anvils, and the anvils will be seated to seal the primers. The primers are then ready for packaging, often in a form that keeps them individually separate so that they are safer for transport and storage by cushioning them against impacts.

Each manufacturer uses their own priming formula, sometimes balancing the compounds to work better based on the type of primer being created and the expected powder type to be used in the cartridges it fires.    

Tips for Handling Primers

Primers are generally safe when handled properly. Here are some tips to keep in mind:

  • Primers are generally safe when handled properly. Here are some tips to keep in mind:
  • Store your primers in their factory packaging until they’re ready for use.
  • Always wear vision protection while reloading ammunition – an exploding primer can drive metal fragments or other particles into your eyes.
  • Make sure you’re always alert and attentive when handling primers – fatigue and distraction can lead to serious, sometimes fatal mistakes.
  • Follow the instructions in your reloading manual to the letter – deviating from these directions can expose you to serious safety risks.

History of Primers: From Match Locks to Modern Ammunition

Back in the 1600s, loading and firing a firearm was a slow, laborious task. First, the shooter poured black powder down the weapon’s barrel, followed by a scrap of fiber wadding and the musket ball itself.

The next step was to pour a small amount of priming powder into an opening at the weapon’s breach called the “touch hole.” Extreme care was needed to keep moisture and dirt out of the mechanism.

Pulling the trigger caused a slow-burning fuse known as the “match” to ignite the powder in the touch hole, setting off a chain reaction that fired the weapon. This entire process could take more than a minute, making each shot precious.

Weapons technology took a giant leap forward in the early 1800s, with the introduction of small, self-contained primers known as “caps.” This eliminated the need to pour the powder into the touch hole, simplifying firearms design and greatly increasing the rate of fire.

By the end of the 19th century, further advances combined primer, gunpowder, casing, and projectile into a single self-contained unit known as a “cartridge.” This led to the invention of modern automatic and semi-automatic weapons.

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