I'm too mathematically inept to figure it out myself, but here's a table showing the diameters of various wire gauges in inches.
wire gauge table
and another one:
another wire gauge table
and one from Reactive Metals:
Reactive Metals conversion charts
According to the chart on the Reactive Metals website, which is the only one that shows decimal equivalents of fractions of an inch, 1/16 of an inch is 0.0625 of an inch, so a bead made on a 1/16 mandrel will accomodate 16 gauge wire (0.051 diameter) but not 14 gauge wire (0.064). Of course, a bead hole will be slightly larger than the mandrel diameter because of the bead release, so 14 gauge wire might fit.
Arrow Springs sells mandrels in .035 and .045 sizes, so it looks like those would fit 20 gauge and 18 gauge wire. I don't think a lot of people use mandrels that small, though. They're tricky to use because they bend so easily.
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