This easy-to-install modular gas block/rail kit replaces the mil-spec front sight/gas block with a modular gas block system which will give you one, two, three or four MIL-STD 1913 rails. All rails will accept Picatinny or Weaver-style mounts. The top rail is integral with the gas block, and the bottom and side rails can be installed or removed in seconds with an Allen wrench. Gas block mounts securely to the barrel (standard .750 inch diameter) with three hardened setscrew, block and rails are one - seven eigths inches long, machined from a 6061 aluminum billet, matte black hardcoat anodized.
Sinclair, Issue:3B, Page:224