If you download the NSRA rules & regulations 1 April 2014 from here;
http://www.nsra.co.uk/index.php/home...y/1-reference#
page 8 refers to scoring in general;
pages 26 - 29 show the method of inward and outward scoring for air rifle and pistol and the sizes of relevant gauges.
Hope this doesn't confuse you too much.
ATB
Mike.