The .222 Remington was the parent case for the 5.56x45/.223 Remington cartridge, and it was also the parent case of the .222 Remington Magnum (which was rejected by the military prior to the adoption of the 5.56x45). The .223 has a longer casing, which holds more powder, as well as differences in neck and shoulder measurements. The two cartridges are NOT interchangeable.
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