This Barrel mounted gas block has a 2 1/2" top rail that is the same plane as standard mil. spec receiver.  The rail is constructed of lightweight hard coat black anodized aluminum.  The gas block weights 2.5 ounces.  The gas block fits over a .625 diameter barrel.  Block locks into place with 2 cross bolts the same as our original gas block front sight.  The picatinny rail is at the AR15 elevation 1.218".