The .223 Remington is a cartridge with the same external dimensions as the 5.56×45
Nato military cartridge. The name is commonly pronounced either two-two-three or
two-twenty-three. It is loaded with a 0.224-inch (5.7mm) diameter jacketed bullet, with the weights ranging from 40 to 90 grains d(2.6 to 5.8g). though the most common loading by far
is 55 grains (3.6g).
While the external case dimensions are the same, the 223 Remington and 5.56×45 differ
in both maximum pressure and chamber throat length. The maximum and mean pressures for some varieties of the 5.56 (different cartridge designations have different standards) exceed the SAMMI maximum for the .223 Remington, and the methods for measuring pressures differ between NATO and SAMMI. 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 can be safely fired in a rifle chambered
for 5.56×45 NATO, firing that ammunition in a .223 Remington chamber may produce pressures in excess of even the 5.56 specifications due to the shorter throat.
NOTE: Many manufacturers of 223 rifles now throat them to safely fire the 5.56 ammunition.
Older 223 rifles should only fire ammunition specified for .223.