Old bbls designed for card wadding usually had the bore area (not dia) reduced to a range of 89-90%, thus √(90%.729²) =.692 or .037" constriction. Plug in any other dia you desire. Later guns for more efficient wadding open this to a reduction more on the order of 92%. This gives a dia of .699" or .030" constriction from the standard .729" for 12ga.

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