NavList:

Nicola B

You ask about sextant calibration certificates.  I suspect the reason none of the experts have waded in yet is because your questions and assumptions raise a long list of “It all depends”.  My experience is mainly with aircraft sextants, but I do have a 1941 Hughes Mates 6” three Circle sextant which I take out and play with regularly, so I’ll put my thoughts in if only to draw out the experts into supplying wiser more experienced replies.  The first thing to realise is that the calibration values on the certificate are effectively how close the markings cut on the arc and any attendant micrometre or vernier system display the true angle subtended at the centre of the arc. These values should not alter much over the life of the sextant.  This should not be confused with index error which effective states how far from exactly parallel rays through the index and horizon mirrors are when the sextant is set to exactly zero.  This can change hourly with temperature or in chunks if the sextant has been subject to a knock or an adjustment.  Defining ‘Practical use’ is the first ‘it all depends’.  If you’re shooting the Sun from a tossing deck looking though the mounting hole only with no scope attached 30” arc error is probably of little consequence in the overall accuracy of any position lines obtained.  At 6AM or PM at the Equator a 30” measuring error would move a Sun line ½ nm, barely the thickness of a 2B pencil line on a typical sea chart.  At 60 N or S it would be half of that.   If on the other had your lying back in a deck chair in your back garden trying to shoot a lunar to correct your chronometer, 30” error would be significant.  In many ways an imperfect arc accurately calibrated is of more use than an arc stated as ‘accurate for practical use’, because you can use the values to correct the former.  The thing is, how accurate was the calibration?  How accurate was the calibrating device?  How experienced and dedicated was the person doing the calibration? 
Regarding magnification.  For fastidious enthusiasts the greater the magnification the better, but for cack-handed operators like myself, the greater the magnification the narrower the field of view and the harder it is to find and keep aligned the astral bodies in the first place.  I manage with 2.5x.  My 5x inverting telescope is way beyond by powers of hand-eye coordination.   However, there’s also resale value.  Mant sextants these day are bought by people who want the flashiest sextant available but who lose interest in using them halfway through the first chapter of the textbook.  6x magnification would probably appeal more than 4x to such people. 

Personally, I’d try and get hold of both and see which I got on with best, even if it’s only how it felt in my hand. That's why it's probably best to visit one of the few remaining sextant shops rather than buying on-line. This shoud bring out some replies for you.DaveP