How to Understand Primers

Winchester Small Pistol Primers for Magnum Pistol Loads

Table of Contents

Introduction to primers

Cartridge primers, though small in size, play a critical role in the functionality of ammunition. Their purpose -- to ignite the propellant powder in a cartridge -- propels the bullet, ultimately deciding the reliability of the ammunition. Understanding the primer, its sizes, types, and numeric ratings, is essential for anyone who reloads. This article provides an in-depth look into the world of cartridge primers.

The inception of the centerfire cartridge was a important moment in firearms history. The .22 BB Cap (Bulleted Breech Cap), also known as the 6mm Flobert, is widely recognized as the first rimfire cartridge, but it was the development of centerfire designs that truly changed ammunition forever.

The Emergence of the Centerfire System

The centerfire system was developed in response to the need for a more reliable, safer, and easier-to-manufacture cartridge compared to rimfire designs. Rimfire cartridges, such as the .22 Short (1857), had the primer compound distributed within the case rim, making them cheap to manufacture but structurally weaker, limiting them to low-pressure loads.

The innovation of the centerfire system moved the primer from the rim to the center of the cartridge base. This meant the cartridge could be thicker and stronger, which in turn allowed for higher pressures, resulting in greater velocity and energy. The first practical centerfire cartridge was developed by Jean Samuel Pauly in the early 19th century. Later, Casimir Lefaucheux introduced the pinfire system (1835), an early centerfire-like design. By the 1860s, true centerfire cartridges emerged.

The First Widespread Centerfire Cartridges

The 11mm Chassepot (1866) used an advanced needle-fire paper cartridge and was one of the most successful early self-contained non-metallic military cartridges, paving the way for later metallic centerfire designs.
The .50-70 Government (1866) was the first centerfire cartridge officially adopted by the U.S. military, preceding the .45-70 Government (1873).
The .577/450 Martini-Henry (1871), used by the British Army, was a centerfire design with a Boxer primer. The original design had a paper-patched .45 caliber lead bullet pushed into a coiled brass case, with a bottle-neck shape that expanded to .577 caliber. It was filled with 85 grains of black powder, producing approximately 1,350 fps muzzle velocity.

Primer Development and the .45-70 Cartridge

While not the first centerfire cartridge, the .45-70 was the first centerfire cartridge to be adopted by the U.S. military, thereby setting a precedent for future ammunition development. Adopted in 1873 for use in the Springfield Model 1873 rifle, the .45-70 was an early adopter of the Boxer priming system, named for its British inventor Colonel Edward M. Boxer.

This primer system, with its central flash hole and a replaceable, separate anvil, was a breakthrough because it was more reliable, safer, and easier to manufacture than the rimfire cartridges that preceded it. The straightforward design of the Boxer primer made it easily removable, making cartridges reloadable -- a significant advantage that led to its widespread adoption in the United States.

Boxer vs. Berdan Primers

Boxer Primer (Colonel Edward M. Boxer, Britain): Features a single flash hole and a built-in anvil. Boxer primers are easier to reload because the spent primer can be easily punched out. Dominant in U.S. commercial ammunition.
Berdan Primer (Hiram Berdan, USA): Features two or more flash holes and an integrated anvil in the cartridge case. Though cheaper to manufacture, they are difficult to reload without specialized tools. Common in military surplus and European ammunition.

Anatomy of a Primer

Despite its small size, a primer is a complex piece of engineering composed of several different parts:

1. Primer Cup: The outer shell that houses all other components. Usually made from brass or copper, it can withstand the firing pin's impact and the subsequent explosion inside the cup.

2. Anvil: A tiny piece of metal positioned within the primer cup. When the firing pin strikes the primer, the anvil provides the resistance necessary for the priming compound to be crushed and ignited.

3. Priming Compound: The chemical mixture that ignites when struck by the firing pin. The composition typically includes lead styphnate, barium nitrate, antimony sulfide, and other chemicals. When ignited, it produces a flame hot enough to ignite the gunpowder within the cartridge.

4. Foil and Paper Disk: Some primers include a small disk made from foil or paper that separates the anvil and the priming compound. This keeps the priming compound in place during assembly and handling.

5. Primer Pocket: While not part of the primer itself, the primer pocket is a key element in a firearm cartridge. It is the recess at the base of a cartridge case that holds the primer. It must be the correct size to accept the primer snugly. For reloading purposes, spent primers are removed from the pocket, which is then cleaned before a new primer is inserted.

Each of these components plays a crucial role in the function of the primer, and therefore, the entire cartridge.

Primer Sizes

Primer size is an essential aspect to consider, as it must match the cartridge case. There are two key categories: small and large.

Small Primers (0.175 inches / 4.45mm diameter):

Small Pistol Primers: Used for standard pistol cartridges (9mm Luger, .38 Special, .380 ACP).
Small Pistol Magnum Primers: Used for magnum pistol cartridges (.357 Magnum) or standard cartridges with heavy loads.
Small Rifle Primers: Used for rifle cartridges (.223 Remington, 5.56 NATO, .17 Remington).

Large Primers (0.210 inches / 5.33mm diameter):

Large Pistol Primers: Used for standard large caliber pistol cartridges (.45 ACP, .44 Special).
Large Pistol Magnum Primers: Used for magnum large caliber pistol cartridges (.44 Magnum, .50 AE).
Large Rifle Primers: Used for larger rifle cartridges (.308 Winchester, .30-06 Springfield).

While this categorization covers most commercial ammunition, some special cases require different primer sizes, such as the .50 BMG (which uses a unique, larger primer). Crimped military primers (found in 5.56 NATO and 7.62 NATO) require crimp removal before reloading. Match primers are specially engineered for precision shooting, ensuring consistent ignition.

Graf & Sons has a handy chart downloadable from their site that details sizes used in many types of ammo.

Primer Types

Primer type directly impacts the performance of the cartridge. There are four primary types:

Boxer Primers: Mainly used in the U.S., characterized by a central flash hole and anvil. Popular due to their ease of removal and replacement, making them ideal for reloading.
Berdan Primers: Often found in military and imported ammunition, featuring two or more flash holes with the anvil forming part of the primer pocket. Generally more cost-effective, but the reloading process is more complex due to their unique design.
Rimfire Primers: Unlike centerfire primers, rimfire primers form an integral part of the cartridge case. The primer ignites when the firing pin strikes the cartridge's rim. Commonly used in small caliber rounds like the .22 Long Rifle.
Shotshell Primers: Exclusive to shotgun ammunition, these come in two sizes: 209 (standard) and 209A (for magnum loads). The 209A variant delivers a hotter, more consistent ignition.

Primer Ratings

Primer ratings indicate the ignition power and the heat a primer generates -- factors that can affect shooting consistency and precision.

Standard Primers: Designed to reliably ignite a broad spectrum of propellants under normal conditions. Suitable for most shooting applications.
Magnum Primers: Generate a longer-lasting, hotter flame, designed to ignite hard-to-burn propellants (e.g., ball powders like H110/Winchester 296) or those used in extreme cold. Warning: Using a magnum primer without reducing the powder charge can increase pressure beyond safe limits.
Benchrest or Match Primers: High-quality primers subjected to stringent quality control for maximum consistency. Typically used in precision shooting, where minor variations in primer performance can significantly impact accuracy.

Primer Numeric Ratings

Specific primer brands, such as CCI, use numeric ratings (like CCI 300 or CCI 250) to classify their primers.

CCI 300: CCI's designation for Large Pistol Primers, ideal for standard pistol rounds.
CCI 250: Large Magnum Rifle Primers. Suited for magnum and heavy load cartridges.

Primer Sensitivity and Reloading Considerations

Federal primers are the softest, igniting with light hammer springs (ideal for tuned revolvers).
CCI primers are harder, preferred for semi-autos and AR-15s to prevent slam fires.
Winchester primers balance between sensitivity and cup strength.

Corrosive vs. Non-Corrosive

Pre-1950s military primers contained potassium chlorate, which left corrosive salts requiring water-based cleaning. Modern primers use lead styphnate, which is non-corrosive.

Understanding the distinctions and functionalities of cartridge primers -- their sizes, types, numeric ratings, and heat ratings -- can drastically influence your shooting performance. Whether you're an enthusiast or professional, selecting the right primer is key for safety, reliability, and precision. Many reloading manuals cover primer information. When in doubt, find a good reference.

Here's a good place to shop for primers.

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