When a gun is fired, the primary ignition mechanism involves the firing pin striking the primer of a cartridge loaded in the chamber. One of the defining aspects of a cartridge is where the primer is located. Whether it is located in the center or in the rim of the cartridge is what differentiates centerfire and rimfire cartridges.
When you look at a rimfire cartridge, the bottom of the casing will look more nondescript than the bottom of a centerfire casing. You won't be able to clearly see where the primer is stored. Instead, the manufacturer of the cartridge actually embeds the primer into the rim of the casing itself. A spent casing will show a firing pin imprint on the outer rim of the cartridge, rather than in the center.
In a centerfire cartridge, the primer is placed in the middle of the base of the casing. The construction of the cartridge's rim is solid, unlike the rimfire shell which has to be folded over in order to fit the primer into the rim. The firing pin strikes the middle of a centerfire cartridge when you fire the gun. If you look at the bottom of a centerfire cartridge, you can see a small, metallic "button" that contains the primer.
When you fire a rimfire cartridge, the cartridge is spent. The rim is dented in order to activate the primer charge. Centerfire cartridges are not typically damaged after you fire them. People who reload their own cartridges can remove and replace the primer "button" in a spent casing, but this is not possible with rimfire cartridges. This provides a potential cost savings on ammunition. However, the sturdier construction of centerfire cartridges makes them cost more upon initial purchase.
Centerfire cartridges generally are more powerful than their rimfire counterparts. Centerfire cartridges are typically larger and stronger. This allows them to sustain more pressure without becoming damaged. This allows manufacturers to create cartridges that develop higher chamber pressure and muzzle energy.
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