Nobody makes bases to fit as many guns as Weaver. Yet each base is machined to tight tolerances for a custom fit. They"re designed to fight the effects of recoil with a square-cut notch that precisely mates to the sturdy cross bolt in every Weaver Detachable Ring. Custom-hardened fastening screws come with every Weaver base.One piece base used on:- H&R: 155, SB2 Ultra (22-250), 925 Topper 158C for 1973 and after; 157 (Drilled and Tapped), Whitetail Unlimited (all calibers)- New England Firearms: Handy Rifle (Use X-High Rings), Hornet (Use X-High Rings), Huntsman Muzzleloader**** Rear sight must be removed to mount the base