The Williams™ 71121 Target-FP  Receiver Peep Sight is specifically designed to fit on an existing receiver sight base. Williams™ Target-FP Receiver Peep Sights offer shooters unmatched open sight visibility at longer distances than standard fixed open sights. The Williams™ Target-FP Receiver Peep Sight is compatible with a front fiber optic fire sight. Williams™ Fire Sights gather ambient light and reflect it back to your eye, enabling faster target acquisition. Williams™ Sights do not use artificial light sources to create their sight visibility, making them legal to use in states that do not allow artificial light sources on your firearm while hunting.