The Leupold steel standard bases and rings are the most versatile of any mounting system we offer. The rear ring is secured by windage adjustment screws, allowing the scope's internal adjustments to be reserved for precise sight-in. Additionally, shims can be used for gross elevation adjustment, reserving the scope's elevation adjustment for bullet drop compensation dial applications. this piece is the base piece for  leupolds classic STD Base-Ring scope mount. The STD Browning A-bolt LR  is a two piece scope base for Leupold STD scope mounting system on Browning Abolt LR rifles.