I think the "True Bore Diameter" term here is an attempt to try to define what the makers use.

"True Gauge" by the formula of what a bore size pure lead ball weighs is 10ga = .775", 12ga = .729", 16ga = .662". 20ga = .615", 28ga = .550" & a .67.49ga = .410". These sizes are recorded by the proof houses.

The old full choke reduction of .040 in a 12 gave an 11% reduction of bore size, a more modern one of .030" gives a reduction of 8%. Lesser constrictions of course give lesser %.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra