XTS Gas Block Black. 750 Diameter Fits AR-15 .223/5.56 NATO Barrel Specifications for XTS Gas Block w/ Top Bottom Rails: Manufacturer: XTS Color: Black Additional Features: .750 Diameter Removable Rails Condition: New Features of XTS Gas Block w/ Top Bottom Rails Aluminum Construction .750 Diameter Fits AR-15 .223/5.56 NATO Barrel Package Contents: XTS Gas Block w/ Top Bottom Rails XTS XTS-GB: Gas Block Black