How a Cartridge Ignites
Rimfire and centerfire describe two fundamentally different ways of storing and igniting a cartridge's priming compound. The firing pin's job is to crush that sensitive compound so it flashes and lights the main propellant, and where the compound sits changes design, durability, and whether a case can be reused. Understanding the distinction explains most of what you see stamped on a case head.
Rimfire Ignition
In a rimfire cartridge, the wet priming compound is spun into the hollow interior of the case's folded rim during manufacture. The firing pin strikes the outer edge of that rim, pinching the compound between pin and chamber so it detonates. Because the rim must be thin enough to crush, rimfire cases are limited to relatively low pressures, which keeps them small.
Centerfire Ignition
In a centerfire cartridge, a separate, replaceable primer is seated in a pocket at the center of the case head. The firing pin strikes this primer directly, and the flash travels through one or more flash holes into the propellant. Isolating the primer lets the case head be made thick and strong, so centerfire designs tolerate the high pressures used in most rifle and defensive rounds.
Reloadability
The location of the priming compound decides reusability. A rimfire case is deformed at the rim when fired and cannot be re-primed, so it is discarded. A centerfire case can have its spent primer punched out and a fresh primer pressed in, which is why handloaders build their practice around centerfire brass.
Boxer and Berdan Primers
Centerfire cases use one of two primer systems. Boxer priming places a single central flash hole and an anvil built into the primer, making decapping simple. Berdan priming uses an anvil formed into the case and two off-center flash holes, which is common in some imported ammunition and more difficult to reload.
Worked Example
Compare the .22 Long Rifle, a rimfire, with the 9x19, a centerfire. The .22 LR carries its primer smeared inside the rim and operates at modest pressure, so its fired cases are simply recycled as scrap. The 9x19 seats a discrete Boxer primer in the center of a thick head, runs at far higher pressure, and its brass can be reprimed and reloaded many times.
A Common Misconception
Many assume rimfire is merely "cheaper centerfire" or that the two differ only in cost. The real difference is structural: the priming location dictates pressure limits, case strength, and reusability. Rimfire is not a weaker version of the same thing but a distinct ignition architecture chosen for small, low-pressure rounds.