The U.S. Optics Fixed Anti-Cant Device (ACD) is a fixed bubble level manufactured from 6061-T6 Aluminum and then Type III hard anodized for durability. The fixed ACD comes with a screw on vial cover to protect the level during storage" but during shooting" the cap can be screwed onto the opposite end's cap saver for safe storage. Mounts to all 1913 picatinny rails.