Appreciating how modern centrefire rounds work helps a long range shooter understand performance and accuracy factors, select the best bullet for the job, and develop the best load for their rifle.

Caveat: I’m not writing from a position of authority. I picked up a new hobby, and writing short articles is a way to document my learning, and encourage further study.

A quick, general comparison of modern ammunition design.

Rimfire:

  • primer built into case rim
  • firing pin strikes rim, sparking primer, igniting powder
  • lower pressures, single-use case must be softer to be crushed
  • usually smaller, lighter, and cheaper
  • easier to manufacture

Centrefire (or Centerfire):

  • distinct primer inserted into centre of case bottom
  • firing pin strikes primer directly, spark ignites powder
  • higher pressures, reloadable case

Cartridge design

Cartridge cross section.svg
By Glrx - Own work based on: Bullet.svg by Quadrell, CC BY-SA 3.0, Link

5. Primer

A small metal cup containing an amount of shock-sensitive explosive chemical. The primer is inserted into a tight recess in the bottom of the case. On pulling the trigger, the firing pin strikes the primer, crushing it against the bottom of the case. A flash hole in the case bottom allows the spark to ignite the propellant.

4. Rim

Part of the case design so the cartridge can be handled by the extractor mechanism after firing. Also used to align rounds in a magazine.

3. Propellant

Unlike the black gunpowder of old, smokeless powder is the norm these days. Contained in the case, once ignited the powder undergoes a chemical reaction that generates an extraordinary amount of gas. It’s this expanding gas that produces the force to propel the bullet out the end of the barrel. Gunpowder is measured in grains, an old unit of mass. 1gr (not to be confused with grams) is approximately equal to 65mg.

2. Case

Commonly referred to as brass (the most popular case material), the case contains and protects the other elements. Depending on their intended use, cartridge cases may have a straight side wall (like the diagram above), or have a neck. Spent cases are retained for reloading. Depending on the quality of both the case, and the firearm chamber, a case may be reusued several times.

1. Bullet

Bullet design has evolved many times over the years, and modern bullets vary wildly depending on their purpose. Bullets are usually defined by a combination of their diameter, and mass (also measured in grains) - the calibre. A common bullet for long range shooting may contain a lead core (softer, higher density, contributing to the bullet mass), with a copper jacket (harder, protecting the core).

Performance

The trajectory of projectiles has encouraged much study. The performance of a bullet in flight is measured by it’s ballistic coefficient - how effectively it slices through the air. While it would make sense to simply choose the bullet with the highest ballistic coefficient, a long range shooter may select one of slightly lower ballistic performance if it’s more consistent for the ranges they plan to shoot. Consistency means predictability - minimising a variable.

Bullet suitability

From the shape, to the materials involved, the purpose of a bullet influences it’s design. Some interesting variations include:

  • hollow-point bullets intended to deform on impact, increasing damage to flesh
  • non-lethal rubber or plastic bullets
  • tracer bullets contain a pocket in the rear for a substance that burns brightly during flight
  • armor-piercing bullets will be high density, very hard, with a pointed tip

Load development

Those deeply invested in precision rifle shooting will tune their rounds to their rifle for specific purposes. Load development involves spending a lot of time understanding and measuring the physics of barrel and bullet interaction, such that you produce the most consistent bullet behaviour. It involves making miniscule adjustments to the mass of gunpowder loaded in the cartridge such that you:

  • optimise the time the bullet spends in the barrel (barrel resonance or barrel harmonics is a thing)
  • optimise the muzzle velocity of the bullet