This one-piece aluminum base is machined to tight tolerances for a custom fit. It offers maximum strength for rings to hold tight. Screws are included. Fits Mossberg 400; Remington 10, 11, 12, 34*, 341*, 510, 511, 512 & 521T. *May require bolt and/or safety alteration, otherwise firearm cannot function safely and properly. Consult with a qualified gunsmith.