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George H, you wrote:
"Has anyone on this list, by measuring star-star distances or by any other
method, ever discovered reproducible errors, outside the terms of a
calibration certificate or maker's warranty, in a sextant? Has anyone made
calibration measurements of his own, in which he has more confidence than in
the manufacturer's scale readings, corrected as necessary by the box
certificate? And if the answer is yes, what's the magnitude of those
errors?"

Sure, I've done this with a few different sextants. Two interesting cases:
I had a Tamaya-alike, assembled by a company whose name I can't remember at
the moment. Using lunars, I made an arc error table that had a maximum value
of 1.8 minutes of arc. The certificate said that the error was insignificant
at all angles. This arc error was reproducible. I eventually sold that
sextant though not because of the arc error. Another, different
Talmaya-alike assmbled by 'International Nautical' has been excellent most
of the time with no measurable arc error, but it has a recurring micrometer
eccentricity problem amounting to as much as 0.8 minutes of arc. By
tinkering with it, I managed to eliminate the micrometer eccentricity at one
point. I don't know exactly how I accomplished this. It was one of those
cases where I took the micrometer apart, put it back together, and it was
fixed. Unfortunately, after a highway trip, the micrometer eccentricity
problem came back (this was a sextant that I let Alex use for a while
 --sadly its perfect performance apparently came to an end just before he had
a chance to try it out). And this latter case, I think, tells us something
important about most modern sextants: they've been banged around enough in
normal shipping that one cannot trust the certificates in the cases. If
you're interested in high-accuracy sights like lunars, you may want to find
a way to calibrate the instrument again.

 -FER

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