The mechanics of this are quite simple. A normal chamber body is a cone having about .005" taper per inch. The forcing cone will have a steeper taper, dependant upon its length. Consider two gauges, one being .001" larger than the chamber's small end & the other .001" smaller. These two gauges will stop at dtstances rather wide apart. The large one will stop a distance of .001/.005 (.200") short. This would indeed show a 2 3/4" (70mm) chamber having a length of only 2.55" (very close to 2 9/16/ or 65mm). On the other hand the smaller one, assuming a difference in chamber & bore of .070" (about standard if both are nominal size) the gauge would enter into the cone .001/.070 times cone length. For a cone length of ½" the gauge would thus go .007" deep, hardly enough to get up on one's tricycle about.
Not having a Galazan gauge I have no idea to what dinension they finish its diameter. These principals though are plain & simple math & apply whom ever makes the gauge.
Many years ago I personally made a set of chamber gauges to precise dimensions of the minimum chamber spec for the small end given in SAAMI (I wanted to be "Precise"). Very rapidly I found out it was not at all uncommon to insert one in the chamber of an older gun & it show to be short chambered, when in fact it was only a little under in diameter.
There will seldom be an issue of a gauge showing the chamber to be too long, even if it was .005" under chamber dia & the bbl had a 1½" cone length the gauge would only go in .107" too deep. It can be readily seen though if the chamber is smaller than the gauge a large discrepency is seen & a chamber can (& WILL) show to be too short when in fact it is only very slightly small.
