A sighting system for a firearm includes a rear sight unit having a plurality of separate sight elements adaptable for "zeroing" the same gun with different ammunition. The multiple-zero sight unit comprises windage and elevation adjustments for each sight element, so that multiple ammunition types having...http://www.google.com/patents/US20040226213?utm_source=gb-gplus-sharePatent US20040226213 - Dual-zero sight for a firearm
