Leupold steel Standard (STD) rings and bases are the most versatile of any mounting system. This specific model fits a Browning BAR and is the perfect choice for standard calibers less than .300 Win. Mag. STD rings come in five different heights and are precision engineered to match up with STD 1-piece or 2-piece bases. The front dovetail ring pivots, allowing for lateral adjustment using the rear windage screws while bore sighting. Shims can also be used, reserving the scope's elevation adjustment for BDC (bullet drop compensation) dial applications. That's why the mounts are as rugged and dependable as the Leupold optics they secure.