One indexing ring comes with your Centra 30mm sight. We have additional rings to enable you to switch your sight from rifle to rifle. Rings are 0.750 inch in diameter and have a protruding stub which mates to a notch in the barrel band of the sight to provide exact alignment. This insures an accurate return to zero when you remount the sight on the rifle.
Sinclair, Issue:3A, Page:210