Micrometer
A measuring instrument more precise than a caliper, reading to a ten-thousandth of an inch through a calibrated screw.
A micrometer measures across a calibrated screw thread, which lets it resolve to 0.0001 inch, an order finer than a typical caliper. The tradeoff is range and convenience: each micrometer covers only about a one-inch span and reads one type of dimension.
Handloaders pull out a micrometer when the last tenth of a thousandth matters, for example sorting bullets by exact diameter or bearing length, or checking the neck thickness of brass before turning it. For most bench measurements the caliper is enough; the micrometer is the referee.