Often erroneously referred to as a ‘Knox Form’, this flat section on the barrel adjacent to the receiver ring on many bolt-action rifles is designed to permit the proper seating of the barrel-wrench during fitting or removal of the barrel.  It performs the secondary function of a reference plane for correctly setting the iron sights.  The Nock's Form is believed to be an invention of the Nock family of 18th and 19th Century London gun-makers, whose name the feature bears to this day.