The .223 Remington is one of the most widely used centerfire cartridges today. Tracing its roots back to the .222 Remington, the .223 was designed to achieve certain ballistics for military use. The .223 Remington, when in its military form, is known as the 5.56x45mm NATO. The differences between the two being the pressures that they are loaded at and the chamber in the firearm itself. When loaded at .223 Remington pressures, Starline's brass is safe to use in any firearm chambered for .223 or 5.56x45mm. If loaded to 5.56 pressures, you need to use firearms labeled for use with 5.56mm. It can be unsafe to fire 5.56mm ammunition in firearms with a .223 Remington chamber.