The XS Big Dot Tritium is the perfect upgrade to a home defense shotgun. Fits Mossberg" Remington" & others with bead mounted directly to the barrel. Big Dot demands your attention - aiding in rapid alignment and indexing on the threat. White dot reflects ambient light for best visibility in low light or shadows" while the center tritium vial glows green in low to no light" making it the perfect front sight upgrade for 24/7 use. The Big Dot epoxies onto the bead (.125"-.140" diameter) with a 2 part epoxy from your local hardware store" i.e.. J-B Weld. Kit comes with replacement bead for Mossberg shotguns" since Mossberg factory bead is too small. Outside Diameter Big Dot 0.188"