Rimfire vs Centerfire: What Is the Difference
Woody's answers are based on published federal and state law. Not legal advice. Consult a licensed attorney for your specific situation.
Rimfire cartridges have primer compound in the rim that gets crushed by the firing pin, while centerfire cartridges have a separate primer in the center of the case that gets struck directly.
How This Affects Your Build
For AR builders, this difference fundamentally determines what platform you're working with. Rimfire ARs use dedicated rimfire receivers and bolt assemblies that can't interchange with standard centerfire components. The firing pin mechanisms, bolt faces, and extraction systems are completely different between the two types.
The Detail
The core difference lies in how the cartridge gets ignited. Rimfire ammunition has primer compound distributed around the inside of the cartridge rim. When your firing pin strikes and crushes this thin rim section, it ignites the powder charge. This design means rimfire cases can't be reloaded since the rim gets permanently deformed during firing.
Centerfire cartridges use a separate primer cup pressed into the center of the case head. Your firing pin strikes this primer directly, creating a more controlled and consistent ignition. The primer cup can be removed and replaced, making centerfire cases reloadable multiple times. This also allows for much higher pressures since the case head doesn't rely on a thin, crushable rim for ignition.
The mechanical differences extend to your rifle's operating system. Rimfire bolts need a much lighter firing pin strike and different timing since rimfire primers are more sensitive. The bolt face geometry is also different, with rimfire bolts having a recessed face to accommodate the thicker rim, while centerfire bolts have a flat or slightly recessed face sized for the primer pocket.
Pressure handling capabilities vary dramatically between the two systems. Rimfire cartridges typically operate at much lower pressures due to the structural limitations of the thin rim design. Centerfire cartridges can handle significantly higher chamber pressures, which translates to higher velocities and more powerful performance.
Build Impact
If you're building an AR-15, you need to commit to either rimfire or centerfire from the start since receivers, bolts, and barrels aren't interchangeable. Most builders go centerfire for the performance advantages and ammunition flexibility, while rimfire builds focus on economical training and plinking applications.