BCM free float Quad Rail eliminates loads from barrel reducing the deflection for more accurate shooting. Precision machined aerospace 6061-T6 aluminum alloy for high strength and lightweight with a proprietary engineered profile offers additional cross sectional strength. Features a steel barrel nut to resist movement under heat. Diameter inside is 1.30″ outside 2.12″. Includes four QD Sing Swivel Sockets and mounting hardware.