U.S. OPTICS BBL100 Rail Mount Fixed Bubble Level The U.S. Optics Anti-Cant Device (ACD) is a fixed or swivel bubble level manufactured from 6061-T6 Aluminum and then Type III hard anodized for durability. The 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 ends cap saver for safe storage. Mounts to all 1913 picatinny rails.