It gets complicated, because the Brits and the Americans (for example--and I expect the Europeans as well) used slightly different chamber diameters. Hugh Lomas once showed me a trick for determining whether an original short chamber was lengthened in the States or England. Slightly different diameter of chamber reamer.
That being said, I have yet to encounter a chamber in which one of the standard Galazan gauges caused me to think it was 2 1/2" when it was really 2 3/4", or vice versa. You may come out a hair off, one way or the other, but not nearly that much.
