I think the best answers are derived from having the most accurate information to work with. Having the actual/accurate bore diameter would help all of us. Variations between manufacturers, in smoothbores and rifle barrels, are not uncommon. My 16 ga. is an early production Getz (~#400) and mikes to .652"
A few years back, we had a pretty good discussion going on "16 ga. loads" and a fairly wide variation was found, in bore diameters, amongst members with "16 ga. smoothbores"
At the time, if I remember correctly, the discussion ran towards the 'popularity' of the 20 ga., which in part, was due to the 'standardization' of the 20 ga. bore diameter amongst barrel makers and the inherent ability of the buyer in easily finding accessories and components. Perhaps the 16 ga. is reaching that point also.
Good luck!