Williams Gun Sight Corporations receiver sight for Marlin 336 and front fire sight for RAMPED FRONT SIGHT . Model 5D receiver sights do not have the internal micrometer adjustments of the FP models. Comes with one 3/8" diameter knurled aperture with a .093" peep. The 5D is strong, rugged and dependable. The blue- black anodizing produces a long lasting no rust finish. The alloy used to manufacture this sight has a tensile strength of 85,000 pounds. Yet, the FP is light and compact weighing only 1 1/2 ounces. Many rifles are now being drilled and tapped at the factory for installation of the FP. Please be aware that some rifles may require drilling and tapping for installation.