"The Four-ten is also known as the 36 gauge ( and 12 mm ).
A simple calculation of the number of lead balls of a certain diameter that can be made from one pound of lead (the usual calculation of shotgun bore sizes) shows us that about 68 balls of 0.410" diameter can be made from 1 pound of lead, hence a four-ten should be a 68 bore/gauge" http://www.fourten.org.uk/36gauge.html