Hello Kevin Cummings
From what you have described in regards to your chamber measurements, "There is a transition point at the outbound end of the chamber"; that indeed is a transition and it is called theForcing Cone. The forcing cone in shotguns was used initially to solve the problem of gas blow-by resulting from the use of felt and/or paper over powder wads.
As you know, the chamber diameter is slightly larger than the bore diameter and on ignition some of the propulsion gases would escape past and around the old fibrous wad material.
Today, many shooter will lengthen the forcing cone, either by reaming and/or lapping them in an attempt to reduce felt (not to be confused with "felt" wadding material) recoil and improve patterns.