Precision Reflex PRi Low Profile Adjustable Gas Block .625 Diameter Black. 625 diameter barrel is constructed of 303 Stainless Steel with a glass bead finish. The gas block is held in place by 2 10-32 set screws. The gas block is 1.3in tall 1in in length and is .625in from bore center to gas tube. Block is .775 in width and comes with a roll pin. From the back of the gas block to the gas port hole is .250. This leaves .035 between the back of the gas block and most steps in the barrel. This is for installing of the A2 style hand guards. So there should be a .035 gap between the barrel step and the gas block. This can be filled with spacer or leave empty. Specifications for Precision Reflex PRi Low Profile Adjustable Gas Block .625 Diameter: Manufacturer: Precision Reflex Color: Black Fabric/Material: Stainless Steel Gas Block Journal Size-Diameter: .625 in Gun Make: AR Platform Package Contents: Precision Reflex PRi Low Profile Adjustable Gas Block .625 Diameter Precision Reflex 05-625-02: PRi Low Profile Adjustable Gas Block .625 Diameter Black