Labgrade, that's a great explanation. I am only familliar with airguages used for barrel measurements, however, rifle barrels aren't cylinders, they have internal geography to them.
Air guages for barrels must be made to the exact rifling twist and bore and groove dimensions for that caliber. Each airguage probe costs quite a bit of money and must be tested for accuracy from time to time to ensure consistency.
It's a pretty complicated system, really, but the actual testing is pretty straightforward. A standard unit with one probe can cost upwards of ten grand.
Hope this helps.
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"We all have it coming, kid..." Clint Eastwood, Unforgiven
[This message has been edited by artech (edited January 28, 2000).]