Marble Arms "Expert" shotgun beads are used exclusively by a number of major shotgun manufacturers. This style of bead can be rib mounted or directly mounted on plain barrel shotguns. This Marble's Expert Shotgun sight is precision machined from solid steel to house the tritium capsule that illuminates your front sight in the dark. This, in combination with a ghost ring rear sight is the perfect combination for law enforcement shotguns and home defense weapons.