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 5.56 x 45 mm NATO. The differences between the two being the pressures that they are loaded at and the chamber in the firearm itself. The .223 has established itself as an excellent varmint and target caliber. It dominates all other rifle calibers in competitions such as 3-gun, and has a dedicated following in several other sports due to its low recoil and accuracy potential. Those two attributes coupled with good velocity allow it to excel as a varmint gun also. When loaded at .223 Remington pressures, our 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 5.56 mm. It can be unsafe to fire 5.56 mm ammunition in firearms with a .223 Remington chamber.