Chuck H;
Excellant post, I will however add one little bit. This is not universal but I have observed (& measured) a good number of bbls which had a slight swelling of the OD at the choke area. Thus if you mic the OD at the muzle & then again about 6" back it is not at all uncommon to find the muzzle dimension to be several thousandths larger than the 6" back measurement.
A lot of guns will measure exactly as you said but quite a few as well will have this enlargement a of the choke area.

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