The .223 Remington was initially developed in the mid-sixties for the Armalite AR-15, which later became the US military's M16 rifle. It is just about identical to the 5.56x45mm NATO military cartridge, and only boffins with measuring equipment could really tell them apart. Or you could just look at the headstamp......
Military adoption has made this a hugely prevalent cartridge in both armed forces across the world and domestic markets due to its devastating performance and lightweight allowing higher capacity magazines and rates of fire. Some in the military feel their rifles should be firing something a bit meatier - however, seeing as it "will penetrate approximately 15 to 20 in (38 to 51 cm) into soft tissue in ideal circumstances" and fragment at high velocities to cause additional internal injuries, it's still not something you will be hoping ever comes your way.
Are you or someone you know in the armed forces? This could be the one for you then....... The longer length, as opposed to pistol cartridges, means you have space for 25 characters so check it out and then get customizing!