Here are some measurements I've recently pulled off the Web. As you can see, they're all over the map.

16 Gauge Bore and Choke Measurements

Briley:
Standard bore measures .662”
Choke Constriction Measurement
Skeet .005” .657”
Improved Cylinder .010” .652”
Light Modified .015” .647”
Modified .020” .642”
Improved Modified .025” .637”
Lite Full .030” .632”
Full .035” .627”
Xtra Full .040” .622”

Patrick Sweeney:
Standard bore measures .667”
Cylinder .000” .667”
Skeet 1 .004” .663”
Imp Cyl. .006” .661”
Skeet 2 .010” .657”
Mod. .015” .652”
Imp. Mod. .025” .642”
Full .028” .634”

Browning:
16 gauge barrels measure .667”
Cyl. .006” .673”
Skeet .002” .665”
Imp. Cyl. .006” .661”
Mod. .010” .657”
Imp. Mod. .024” .643”
Full .043” .624”

Other:
Standard bore measures .667”
Cylinder Bore .000” .667”
Skeet I .004” .663”
Improved Cyl. .007” .660”
Skeet II .010” .657”
Modified .015” .652”
Improved Mod. .020” .647”
Full Choke .028” .639”
Extra Full .035” .632”

L.C. Smith:
Standard bore measures .662”
IC .005” .657”
Mod. .015” .647”
Full .033” .629”

The only constant in life is change.