The .223 Remington is a cartridge that is ballistically in between its predecessors, the .222 Remington, and the .222 Remington Magnum. The .223/5.56 was developed to fit the action length of the popular M16 service rifle. The .223/5.56 quickly became popular as a civilian cartridge because of the availability of brass, and the chambering of commercial varmint rifles in that caliber. Shortly after military acceptance of the M16, the semi-automatic version, the AR-15 became available, making the .223 cartridge even more popular.
The .223 Remington is a cartridge with almost the same external dimensions as the 5.56 NATO military cartridge. It is loaded with a 0.224 in. diameter jacketed bullet, with weights ranging from 40 to 90 grs, though the most common grain bullet used today is 55 grains.
While the external case dimensions are very similar, the .223 Remington and 5.56 NATO differ in both maximum pressure and chamber shape. The maximum and mean pressures for some varieties of the 5.56 (different cartridge designations have different standards) exceed theSAAMImaximum for the .223 Remington, and the methods for measuring pressures differ between NATO and SAAMI. The 5.56 chamber specification has also changed since its adoption, as the current military loading (NATO SS-109 or US M855) uses longer, heavier bullets than the original loading. This has resulted in a lengthening of the throat in the 5.56 chamber. Thus, while .223 Remington ammunition may be safely fired in a rifle chambered for 5.56 NATO, firing 5.56 ammunition in a .223 Remington chamber may produce pressures in excess of even the 5.56 specifications due to the shorter throat.