Leupold Standard Browning A-Bolt Shotgun Steel Scope Base - STD Browning A-Bolt Shotgun 2-pc. The forward part of the base accepts a dovetail ring, locking it solidly into position. The rear ring is secured by windage adjustment screws. 2-Piece bases fit right and left-hand actions, and reversible bases are available for many actions. Mark 4 Tactical Bases are the only bases to accommodate both the oversized 8-40 screw and a 6-48 screw, providing compatibility with the widest range of modern tactical firearms. Both styles of screws are included with each Mark 4 base.