Precision Reflex AR15/M16 Gas Block w/ Top Picatinny Rail Black. Is designed for a .750 diameter barrel and has a 2 1/2in top rail and weighs 2.5 ounces. The gas block is held into place with 2 #10-24 screws. Specifications for Precision Reflex AR15/M16 Gas Block w/ Top Picatinny Rail: Manufacturer: Precision Reflex Color: Black Fabric/Material: Aluminum Gun Make: AR Platform Package Contents: Precision Reflex AR15/M16 Gas Block w/ Top Picatinny Rail Precision Reflex 05-075-04: AR15/M16 Gas Block w/ Top Picatinny Rail Black