The problem is that those diameters have varied depending on when the gun was proofed. For example, per Baron Engelhardt, by the British Rules of Proof of 1904, still in force as of 1951, a British 16ga could vary in diameter from .637-.669. But if it measured .637, it would have been marked as an 18; if .669, as a 16/1. There are 5 different "gauges" within that spread, even though they all would have been chambered as 16's. And each of those separate gauges has a range of bore diameters within which it is in proof, for that gauge.
Likewise, per Hunter Arms drawings included in Brophy's book, LC Smith 16ga bore diameter at one time (date not included on the drawing) was .650.
