Testing was done with a Remington 722 rifle with a 22" Apex barrel.
During a public demonstration the round successfully penetrated the US steel helmet as required, but testing showed chamber pressures to be excessively high.
In 1964 the .223 Rem cartridge was adopted for use in the Colt M16 rifle which became an alternate standard rifle of the U. In 1980 NATO modified the .223 Remington into a new design which is designated 5.56×45mm NATO type SS109. A project to create a small caliber high velocity (SCHV) firearm was created.
The development of the cartridge which eventually became the .223 Remington was intrinsically linked to the development of a new lightweight combat rifle. Eugene Stoner of Arma Lite was invited to scale down the AR-10 (7.62 mm) design. Springfield Armory's Earle Harvey lengthened the .222 Remington cartridge case to meet the requirements. Concurrently with the SCHV project Springfield armory was developing a 7.62 mm rifle.
Many AR rifle owners choose to build their own rifles, which is facilitated by a huge variety of barrels and other components.
The custom built AR may have a barrel from 7.5" (which is classed as a pistol) to as long as 24" used in varmint rifles primarily, often with Wylde or Noveske chambering.
At an Independence Day picnic air force general Curtis Le May tested the AR-15 and was very impressed with it.
Eugene Stoner of Arma Lite (a division of Fairchild Industries) had been advised to produce a scaled down version of the 7.62 mm AR-10 design.The cases tend to have similar case capacity when measured, with variations chiefly due to brand, not 5.56 vs .223 designation.The result of this is that there is no such thing as "5.56 brass" or ".223 brass", the differences in the cartridges lie in powder loads which are affected by variations in case capacity. When handloaded, care is taken to look for pressure signs as 5.56 mm cases may produce higher pressures with the same type of powder and bullet as compared to .223 Rem cases.Because of several different .222 caliber cartridges which were being developed for the SCHV project, the .222 Special was renamed .223 Remington.In May 1959 a report was produced stating that five to seven man squads armed with AR-15 rifles have a higher hit probability than 11 man squads armed with the M-14 rifle.and is used by most makers of complete rifles and components.Many AR (Armalite) type rifles use 1:9, which is suitable for bullets up to 69 grains or 4.5 grams or 1:7, which is suitable for bullets up to 85 grains or 5.5 grams.Case capacities have been observed to vary by as much as 2.6 grains.Sierra provides separate loading sections for .223 Rem and 5.56 mm NATO and also recommends different loads for bolt action rifles as compared to semi-automatic rifles.A variety of .223 Remington cartridges and a .308 Winchester (right) for comparison.Bullets in .223 cartridges (left to right): Montana Gold 55 grain full metal jacket, Sierra 55 grain Spitzer boat tail, Nosler/Winchester 55 grain combined technology, Hornady 60 grain V-Max, Barnes 62 grain Tipped Triple-Shock X, Nosler 69 grain hollow point boat tail, Swift 75 grain Scirocco II.