Case capacity is about 5% smaller than that of the .222 Remington Magnum, but it was loaded to a slightly higher pressure, so the two have essentially identical ballistics. As any widely used military cartridge is guaranteed to be a success on the commercial market, the .223 Remington sold exceptionally well and the .222 Remington Magnum faded rather quickly.
The .223 Remington cartridge has a slightly smaller capacity than the .222 Remington Magnum, but it is loaded to a higher pressure, resulting in similar ballistics.  As any widely used military cartridge is guaranteed to be successful on the commercial market, the .223 Remington sold exceptionally well and the .222 Remington Magnum quickly lost popularity.