Abstract: The present invention relates to improvements in a power rifle scope of the type having a tubular housing barrel with enlarged end portions and an optical axis defined by an optical system including an objective lens disposed in the enlarged front end thereof, an eyepiece lens system disposed in the enlarged other end thereof and an erector lens system intermediate the said two ends and where the improvement comprises the addition of a variable optical aperture mounted in and carried by a cylindrical guide tube which is secured interiorly of the scope barrel, a helically rotatable setting ring carried by and disposed exteriorly of the barrel and having an inwardly radially directed actuating pin which passes through the barrel and through a circumferential slot in the guide tube and operatively engages the variable aperture device to vary the light passing capacity of the barrel in response to rotation of the setting ring and sealing rings disposed between the setting ring and the barrel, for and aft of the a