Sub-caliber upper assemblies allow fast and simple conversion of ARs chambered in .223 to 9mm Luger. True, dedicated, complete upper receivers are assembled from military specification components; includes a Wilson, 1-10 twist barrel, bolt and carrier, charging handle, shell deflector, heat-shield lined handguards, sling swivel, and A1-style flash hider. Additional items not included but required for conversion to 9mm are a 9mm magazine (078-000-320); magazine adapter, action spring, and recoil buffer (739-000-016). A4 unit has flattop receiver that accepts most aftermarket accessories designed to fit a Picatinny rail. Mfg: Rock River Arms SPECS: Barrel – 4140 steel, blue, matte finish, 16″ (40.6cm) long, .725″ (18cm) O.D. Receiver – Aluminum, anodized finish, matte black. Designed to fit lower receiver with .25″ (6.3mm) diameter takedown pins.