What I have always gone by is 16ga:

bore dia US....16.89mm or .6650in(min) & 17.40mm or .6850in (max)

Chokes:
.000 - .002 cylinder
.004 - .012 imp cylinder
.016 - .024 modified
.028 - .032 imp. mod.
.036 - .039 full

this has always been a good reference for me.

tim