The .223 Remington was the cartridge from which the 5.56mm NATO was developed. The 5.56mm NATO is loaded to higher pressures, as all NATO rounds are, and are said to not be safe fired from guns specifically designated for the .223 Remington cartridge. You can fire .223 Remington rounds out of a gun designated for 5.56, as it is the higher pressured cartridge and therefore the gun built for it has higher tolerances. They are the same, but then again, they arent.
This can also be said for the .308 Winchester and the 7.62mm NATO round.
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