Weaver's Top Mount Base is machined to tight tolerances for a custom fit. It offers maximum strength for rings to hold tight and is constructed of aluminum. Fits BSA Monarch medium action in 222, 243 & 308, long action in 7mm, 270 & 30-06, CF2; Churchill Highlander; Rem 721; Herters U9 round receiver; Howa 1500; Mossberg 4X4, Mossberg 100 ATR, 1500, 1700, MVP; Raptor Arms; Rem 700, 78, 40X-L, 721 & 725 (long actions), 700, 78, 40X-S, 722 & 725 (short action);S&W 1500, 1600 and 1700; Weatherby Vanguard, Mark V standard caliber & 240MAG, Centerfire pistol, 224 Varmintmaster, Ultra Lightweight.