Bore Gauge questions

[Cold Spring]

I have no doubt as to the accuracy of Austin's measurements and drawings for those specific guns but I often wonder when I see a "swell" (short enlargement) of the bore as it starts into the choke leade. Austin shows typical ogees but not swell profiles. Anyone who's measured enough shotgun bores with an accurate bore mike has seen that swell on various vintage makers and to include some earlier/modern fixed choke guns, for example Winchester 101's. Typically on heavier choked barrels and supposedly to act as kind of a short run overbore for bare shot.