>>27736350>.223 Remington was a varmint hunting round before it was considered for adoption as 5.56.The '.222 Special' was developed from the .222 Remington in 1957 as part of the US government's SCHV program. It was renamed '.223 Remington' in 1959. The AR-15 and its .223 cartridge began USAF trials in 1960. It was not offered commercially until 1962. It was officially adopted as the 5.56x45mm Ball, M193 in 1963.
An earlier round developed by Springfield Armory, the .224 Springfield, was sold commercially as the .222 Magnum.