10 ga has a nominal rim of .933" and a head diameter of .854"
8 ga is not legal for hunting in the USA and therefore not listed by SAAMI.
Per Dave Cushman, the chamber rim recess is 1.035" so the shell rim could be no larger, likewise the main chamber diameter is .930 so the case head would be a bit less. I might GUESS 920" based on 10 ga numbers.