What is rimfire? How rimfire ignition works vs centerfire

Written in our own words from the sources cited after each fact. We haven't run hands-on tests for this page; where sources disagree we say so. Prefer the primary source (SAAMI, the maker, the governing body) if anything here conflicts with it.

The short version

A rimfire cartridge has no separate primer. The impact-sensitive priming mix sits inside the hollow, folded rim at the base of the case, and the firing pin fires the round by pinching that rim against the edge of the chamber^{[NIJ firearms examiner training]}^{[Shooting Sports USA]}. A centerfire cartridge instead carries a small primer cup in the middle of the case head, and the pin strikes that cup^{[Shooting Sports USA]}.

Step by step

At the factory, a wet pellet of priming mix goes into the empty case, and a spinning tool forces it outward into the rim cavity. American Rifleman puts the spin at roughly 10,000 rpm^{[American Rifleman]}. NIJ's training module describes the same charge-plate-and-spinner process^{[NIJ firearms examiner training]}.
The mix is dried. Rimfire mixes may include a frictionator such as finely ground glass to help ignition^{[NIJ firearms examiner training]}^{[American Rifleman]}.
Powder and bullet are added and the case mouth is crimped^{[American Rifleman]}.
When fired, the pin crushes the rim, the mix ignites, and the flame lights the powder. Ignition starts on one side of the case because there's no central flash hole^{[American Rifleman]}.

Why rimfire stays low pressure

The rim has to be thin enough to crush, which also makes it the weak point of the case. SAAMI's rimfire standard sets a Maximum Average Pressure of 24,000 psi for .22 Long Rifle^{[ANSI/SAAMI Z299.1-2026 (rimfire)]}. American Rifleman ties that ceiling directly to the thin case head^{[American Rifleman]}. That's why rimfires are limited to small calibers, today mostly .17 and .22^{[Shooting Sports USA]}.

Why you can't (practically) reload rimfire

The fired rim is dented and the priming is part of the case, so there's no primer to swap out. American Rifleman notes reloading rimfire needs specialized, expensive tooling, which keeps most handloaders away^{[American Rifleman]}. Hobby kits do exist for re-priming .22 WMR cases, but they involve mixing impact-sensitive compound by hand and there's no published load data from mainstream sources^{[Shooting Sports USA]}. We don't recommend it. CZ's manual calls reloading an expert activity and tells owners to use quality commercially made ammunition^{[CZ 457 instruction manual (03/2024)]}.

Rimfire vs centerfire at a glance

	Rimfire	Centerfire
Primer	Mix inside the folded rim^{[NIJ firearms examiner training]}	Separate cup in the case head^{[Shooting Sports USA]}
Pressure	.22 LR MAP 24,000 psi^{[ANSI/SAAMI Z299.1-2026 (rimfire)]}	Much higher in most rifle cartridges
Reloadable	Not practically^{[American Rifleman]}	Yes, commonly

Next: the rimfire cartridge family · history · misfires and dry firing.