There seem to be divergent opinions on the correct diameter of 16 gauge.

Boxall and Edmiston feels that it is .670"
http://www.boxallandedmiston.co.uk/shotg...ghtly-different

Wikipedia has it as .663"
https://en.wikipedia.org/wiki/Gauge_(bore_diameter)

Hallowell at .662"
http://www.hallowellco.com/bore_size_chart.htm

World Almanac and Book of Facts at .660"
https://www.unc.edu/~rowlett/units/scales/shotguns.html

International Hunter Education Association at .670"

forensicmed.co.uk at .665"

And so on.