Kensal makes an interesting and "accurate" point about determining 'choke" and not having to zero the instrument prior to use. Since choke is found by the difference between bore and muzzle diameters, assuming that the dial is firmly snugged and does not drift/alter setting it matters NOT that the gauge be pre-zeroed.
However, I like to know EXACTLY my bore diameter in various locations, so I zero the gauge prior to each session. (The passage of the three balls thru the bore also tells me the smoothness or roughness of the bore by FEEL as I push/pull the gauge shaft.)
And since the palest ink is better than the strongest memory, I make a dated entry of the measurements in the chapter for that particular firearm in my Firearms Log....along with any other work I did on that firearm on that particular date.
I have the 12" version of Skeet's Bore Gauge - and its only short-coming is that I can't measure the center 6" of a 30" barrel.

Last edited by Ian Nixon; 02/21/12 12:16 PM. Reason: Gauge length short-coming.