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 impact-sensitive explosive material 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 spark through a flash hole in the cartridge case and igniting the main powder charge.
The combustion of the gunpowder begins a chemical reaction that transforms it into rapidly expanding gasses that create the necessary pressure to push a bullet through the barrel of your gun.
In many ways, the primer in your pistol cartridge works on the same principle used in demolition and mining operations. 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:
- Rimfire Primers – the primer is integrated with the rim of the cartridge and is not a separate component. This type of cartridge is not commonly reloaded because of its design
- 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 most common primers are available in 4 different primer sizes, typically broken down as:
- Small rifle primers – typically .175 inches in diameter and .120 inches tall.
- Small pistol primers – designed specifically for handguns, these are approximately the same size as small rifle primers.
- Large rifle primers – these measure approximately .128 inches in height and .212 inches in width.
- 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.
An anvil— a small protrusion or bump located at the center of the primer cup— will be seated to ensure the firing pin creates enough pressure to crush the priming compound and ignite the gunpowder. Finally, another sheet of metal will be stamped to create covers for the primer cups.
The primers are then ready for packaging, often in a form that keeps them individually separated and cushioned so they are safer for transport and storage.
It’s important to understand each manufacturer uses their own priming formula. This often involves balancing the compounds to enhance performance according to the specific type of primer being produced and the intended powder type for the cartridges it will ignite.
Tips for Handling Primers
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 – pouring them into a separate container can cause them to detonate.
- Always wear vision protection while reloading ammunition – otherwise, an exploding primer can drive metal fragments into your eyes.
- Make sure you’re always alert and attentive when handling primers – fatigue and distraction can lead to serious, sometimes fatal mistakes.
- Avoid using ammunition manufactured during or prior to World War II – primers from this time period might contain dangerous corrosive chemicals. For the same reason, you should avoid ammunition made in Russia or China.
- 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 gun 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 powder into the touch hole, simplifying gun 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.