Back in the late 1950s, and early sixties, when the .222 Special was being developed by RA, the US Military was changing over to “Metric Nato Designations” for all new Ammunition.
The Initial description of the “New Cartridge” was [b]Cart. Cal.5,64mm /b as seen on some early manuals for the New, AR-15 auto rifle.
This was the exact metric designation of the Bullet diameter (.224), and NOT the Bore diameter of the Rifle, which was .219 ( or 5,56mm)…somebody at either Remington or Armalite ( American) did not understand the “European Metric Bore diameter military DESCRIPTION” system. Soon after that, the ‘cal’ was corrected to 5,56mm…in correct military terms
Remington, of course, saw it had a “New Cartridge” available, with more power than the “222 Rem” and equal to the “.222 Rem Magnum” (ie, 5,56x47), but with a shorter case…There was also the “.224 Winchester”, a competitor cartridge for the “Light Rifle project” of Springfield Armoury, running concurrently with Stoner’s Armalite program.
So, taking a Middle Road, Remington called their cartridge, the “.223 Remington”, and settled the specs for a Commercial sporting case. (Thinner Brass, different tolerances at the Neck, etc; at the time, it was meant to be a “Bolt action” cartridge…Ruger Mini-14s ad AR15s in semi-only were still a few years away.
The Military, of course, wanted a “hot round” (penetrate a steel helmet at 600 metres) and reliability in Full-Auto operation…( we will leave out the Ball Powder-Tubular Grain Powder Fiasco in early Vietnam)
So the Specs for the Military cartridge ( “5,56mm”) were different both in Cartridge and Chamber design.
Hence the perceived and real problems of using Milspec ammo in sporting firearms for the same cartridge design.