Mark 4 Tactical Bases are the only bases to accommodate both the oversized 8-40 screw and a 6-48 screw, providing compatibility with the widest range of modern tactical firearms. Both styles of screws are included with each Mark 4 base. Securely attach your scope to any M16 variant with a carrying handle using this rugged mount. Delivers rock-solid stability for repeatable accuracy. Use with Leupold QRW mounts.