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 are the pressures that they are loaded at and the chamber in the firearm itself.
The .300 Blackout is based on a shortened 5.56x45mm case that is necked to .30 caliber. For AR-15 platform rifles and pistols, changing the barrel only requires going from 5.56 to .300 Blackout. The Blackout's main purpose is to be used at subsonic velocities in suppressed firearms. Many bolt action and single shot rifles are now being produced in this caliber as well.