Many makers bored their barrels to other than nominal size for the gauge. Some were over while others were under. If you look in the proof house specs though you will see the nominal bore for the 16 is .662". This is based on the volume of a .662" diameter ball of pure lead will weigh 1 ounce or 16 to the pound. Matters not what any individual maker bores their barrels to, this is the nominal gauge size.

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