Fits Mossberg, Remington & other shotguns with a bead mounted directly to the barrel. The white dot reflects light for increased visibility in poor lighting situations. The center tritium vial glows green in low to no light, making it ideal for 24/7 use. This sight glues over existing front beads that measure 0.120”-0.140” in diameter. May require opening up bead to larger size by drilling and tapping. Mossberg factory bead with hex base must be replaced. The outside diameter of the Big Dot is 0.188”. XS Big Dot is a standalone solution and does not work with any rail or ghost ring solution. Includes a conversion bead for Mossberg shotguns.