.223 Remington

The .223 Remington is a cartridge with almost the same external dimensions as the 5.56×45mm NATO military cartridge. It is loaded with a 0.224-inch (5.56 mm) diameter jacketed bullet, with weights ranging from 40 to 90 grains (2.6 to 5.8 g), though the most common loading by far is 55 grains (3.6 g). While the external case dimensions are very similar, the .223 Remington and 5.56×45mm differ in both maximum pressure and chamber shape. This allows for .223 Remington ammunition to be safely fired in a rifle chambered for 5.56×45mm NATO. The reverse however, is not possible. [Wikipedia]