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. This common and easy-to shoot cartridge is ideal for defense, training and small-medium game hunting.