1 inch = 25.4 mm thus;
10 gauge = .775" = 19.69 mm (.775 x 25.4)
8 gauge = .835" = 21.21 mm (.835 x 25.4)
Both rounded to two decimal places.

A few smokeless loads using "Slow" powders loaded light, down to pressure levels which were never intended by their makers, can produce lower chamber pressure than black. These loads will burn "Dirty" & may result in squibs.
When smokeless is loaded as intended it will give higher chamber pressures than Black.

A 16 gauge with three inch chambers is relatively rare, but not unheard of. The vast majority of them are most likely pre-WWI.

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