.223/5.56 refers to two closely related rifle cartridges—.223 Remington and 5.56 NATO—commonly used in AR-15 platforms. While not a manufacturer, many firearms companies produce .223/5.56-specific parts, including barrels, upper receivers, bolt carrier groups, magazines, and complete rifles or pistols. These components are popular for their versatility, reliability, and performance in target shooting, hunting, home defense, and tactical applications. If you need info on a specific manufacturer of .223/5.56 parts, let me know!