Leupold steel Standard (STD) rings and bases are the most versatile of any mounting system we offer. This specific model fits a Ruger American RF (Rimfire) 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.