Though some people want to call these two distinctly different ctgs. I'd remind everyone that there is only one ctg drawing & only one reloading die. The ctg was invented by Remington at Armalites request. It was originaly called the 222 Remington special. At that time Remington had 2 different 222 Remington ctgs, The 222 Remington & the 222 Remington magnum. The original AR15 was chambered for 222 Remington. The Military changed the spec from desired dbl penetration of a GI helmet @ 300 yds to 500 yds ( Some say they thought that was an unobtainable goal & would eliminate the unwanted AR, as they were quite happy with the M14 at the time & were being forced against their will to test the AR).The 222 Remington was capable of that @ 300 but not @ 500. The 222 Remington magnum was capable of it @ 500 but was too long for the already existing AR magazine. So they split the difference between the two & came up with a 3rd 222 Remington ctg, the 222 Remington special which later became the 223 Remington in SAMMI specs, perhaps to avoid confusion, since they already had two 222 Remington ctgs & a 3rd would only increase confusion. At this time Remington would not have touched a Metric designation with a 10 foot pole. They had recently introduced the 6mm Remington & Winchester had introduced the very simular 243 Winchester. The Winchester rd though technically inferior too the 6mm Remington became a great success while the 6mm Remington floundered. Remington concluded probrably correctly that the American shooter didn't want no durn weird metric ctg. The US Army intially called this ctg the 222 Special also, but was shortly going through a metric conversion that was supposed to completly eliminate the English inch system in the US ( didn't happen despite the Govts best efforts), as a federal entity the US Army had no choice but too comply with orders to convert to the Metric system, it also has a limted amount of logic since most of our allies including the English have converted too the Metric system. So the official moniker the Army ended up giving the new ctg was 5.56 NATO. That doesn't mean they are complety different ctgs. M9