Gives an instant check to make sure the gun is perfectly level for every shot, so every shot goes where it's aimed. If you sighted in carefully at 100 yards with the crosshairs level, then canted the gun just six degrees you'd move the next shot over 1/2''''. Not much, but enough to completely miss a small target, or wound a larger one. The problem gets worse at longer ranges. Clamp the ACD on the scope tube where it's visible to the non-sighting eye. Mfg: Mounting Solutions Plus SPECS: Aluminum. Black or silver, anodized, matte finish. Available for 1'''' or 30mm scopes.