While a small hole gage is admittedly spherical it also has to be small enough to go in the smallest hole of its range. It too will measure two points 180 apart. It will thus give a false reading with an odd number of lands & grooves. Depending on the width of the grooves it may or may not reach the bottom.
The "Original" question in this thread included the "40" mark on the barrels. This mark applies to the "Bore" diameter, thus the ensuing comments on that subject.
Technically speaking the caliber of a rifle is its bore size. No doubt, in this case, the 40 was the correct marking under the old Gauge system. Makers have taken such liberty in naming their creations that in many cases neither bore nor bullet diameter is actually described by the cartridge's name. A couple of examples which immediately spring to mind are the .44-40 & .38-40 Winchester cartridges.
