Chambers: 5.56 NATO/223 Rem., 223 Wylde, 6 ARC, 300 BLK. 5.56 Barrel: 16", 1:7" twist, or 16" 1:8" 223 Wylde Hanson-profile stainless steel. 6ARC 18" 1:7.5" twist Proof Research full profile. Thermal fit barrel, precision billet upper with flat barrel seat, ArmorLube BCG. Proprietary buffer system with flat wire technology. The lower receiver has a right-side bolt release, flared mag well, and an E series Cerakote finish. GWOT (Global War On Terror) has a 16" full-profile barrel. The SOIDC (Special Operation Independent Duty Corpsman) variant has a Proof Research carbon-fiber-wrapped barrel. Add 35% for the SOIDC Model.