Each Weaver base is machined to the tightest tolerances for a custom fit. They are 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. - Fits: - Mossberg 400 (Front) - Remington Models 10 11 12 34 341 511 512 & 521T (Front)