Rimless in the context of .223 Remington

The .223 Remington, also known as 223 Remington by SAAMI and 223 Rem. by the C.I.P., (pronounced "two-two-three") is a rimless, bottlenecked, centerfire intermediate cartridge. It was developed in 1957 by Remington Arms and Fairchild Industries for the U.S. Continental Army Command of the United States Army as part of a project to create a small-caliber, high-velocity firearm. Firing a .2245 in (5.70 mm) projectile, the .223 Remington is considered one of the most popular common-use cartridges and is used by a wide range of semi-automatic and manual-action rifles.