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.