5.56x45mm NATO / .223 Remington
Cheap, soft-shooting, and everywhere — the AR-15's cartridge and the modern American default, with the .223/5.56 chamber distinction worth knowing but rarely a problem in practice.
Family
Rifle
Introduced
1963
Origin
Remington and Eugene Stoner's ArmaLite team, USA
Military
M193 ball; M855/SS109 5.56 NATO; M855A1 in current US service
History
Developed in the late 1950s from the .222 Remington family for the ArmaLite AR-15, the .223 Remington went commercial in 1964 just as the US Army adopted it in the M16. NATO standardized the heavier-bulleted SS109/M855 version in 1980. As the AR-15's cartridge it became the highest-volume centerfire rifle round in America.
What it's used for
Iconic platforms
The AR-15 and M16/M4 family, Ruger Mini-14, and bolt guns from nearly every maker; 5.56-chamber rifles also shoot .223.
Common load types
FMJ ball, match hollow point, polymer-tip varmint, soft point, and bonded defensive loads.
Reference only — not loading data, not ballistics advice, and KitID links to no ammunition sellers. Know your local law before you buy anything that goes bang.