.000010" (ten millionths of an inch)resolution capabilities. Very precise measurements can be taken.
A precision "handle" with air blowing through it is zero'd in a known size ring (say .038" inside diameter for a .308 cal bbl). The "handle" is then inserted through the bore & deviations from the ring's known zero (inside size of the ring) are viewed, on a dial, as + & - from that known size. Works off of back pressure of the air. The bigger the bore (relative to the zero) allows more air to pass (less back pressure) so shows up on the dial on the + side of the dial. Smaller bore diameter = higher back pressure = - side of the dial.
'Bout the extent of it. Above's a pretty simplistic explanation. If you really want more, I'll do it in more detail.
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