The .223 Remington (5.56x45mm), also known as .223 Rem or .223, is a small-bore, high-powered, intermediate rifle cartridge. The bullet is approximately .224 inches (5.56 mm) in diameter, and 45 mm in length.
The .223 Remington cartridge is visually nearly identical to the 5.56 NATO cartridge, with the exception of the casing headstamp, which usually bears the stamp of whomever manufactured the ammunition. It can be fired safely in the same weapons as the 5.56 cartridges, as the .223 is loaded to a lower pressure.
Heavier .223 bullets tend to be longer, while lighter .223 bullets tend to be slightly shorter.
The .223 chamber has a shorter and sharper leade compared to the 5.56 chamber, as well as less effective freebore.  Firing 5.56 cartridges in a .223 chamber will cause the pressure to spike due to the projectile engaging the rifling sooner than it would in a 5.56 chamber. Use caution when firing 5.56 NATO cartridges in a weapon suspected to have a .223 chamber; common symptoms are popped primers and gas staining on the primers. Contrary to popular belief, the .223 cartridge casing does not have thinner case walls than the 5.56 case; there is no discernible difference, though the capacity will vary slightly due to manufacturing tolerances, and 5.56 casings can be loaded to .223 pressures.
The .223 has many variations. It can be loaded with the same projectiles as the 5.56, as well as soft point projectiles, hollow point projectiles, frangible projectiles, and others; the only difference would be the lower pressure and a slight loss of muzzle velocity. The bullet weights range from around 26 grains (frangible) to around 80 grains; heavier bullets exist in this caliber, but are only used in bolt-action rifles, as the OAL is too long for most magazine-fed weapons.
This cartridge is generally fed through box magazines.