The MBUS Pro (Magpul Back-Up Sight) is a corrosion-resistant all-steel back-up sighting solution that delivers maximum functionality and strength with minimum bulk at a price that&##39;s even smaller than its size. Case-hardened and finished with Melonite QPQ for extreme resistance to wear and corrosion. The MBUS Pro rear sight is windage adjustable with low profile know, no tools required. Positive detents keep the MBUS Pro Front and Rear at standard AR sight height when deployed, and so low when they are stowed that you&##39;ll never notice them until they&##39;re needed. Small aperture diameter: 0.07 inches. Large aperture diameter 0.19 inches. 0.5 MOA (0.566"/100m) per click with a 14.5" sight radius. 0.4 MOA (0.410"/100m) per click with a 20" sight radius. Made in the USA.