A rear gun sight mountable to a firearm that includes an aperture module defining a gun sight aperture and a base. The base defines a recess for receiving the aperture module and the aperture module includes ribs that are slidably received by associated recess in the base to constrict the aperture module to relative rectilinear movement with respect to the base. At least one spring acts between the base and the aperture module. An adjustment member couples the module to the base such that rotation of the adjustment member moves the module towards and away from the base depending on direction of rotation. A detent resists uncontrolled rotation of the adjustment member. The aperture module includes a recess for receiving a light emitting member such as a tritium module.