You would most likely loose the ability to use pressures above 2400 psi with the HDD.
Your right the best way to measure a small hole is with a pin gauge but I can come within .001-.0005 with a good set of calipers. Calipers have two sets of jaws on them one for measuring inside measurements and one for outside. I'm an engineer I measure things for a living. Holes only become a challenge to measure with calipers once they become smaller than about .065
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