5.56 NATO was developed in the United States and initially chambered in the M16 rifle. It is the standard cartridge for NATO forces as well as many non-NATO countries. It is derived from, but not identical to, the .223 Remington cartridge. We use a NATO, mil-spec chamber in our barrels that is reliable and safe to fire both 5.56 and .223 Remington rounds. Enhanced NATO chamber is accurate and reliable even in high round count shooting. This common and easy-to shoot cartridge is ideal for defense, training, and small-medium game hunting.