If you want to be anal about it, paint your gauge with clear nail polish until it reaches the exact chamber diameter. However, it wouldn't be right or possible to use it that way if you tried to measure a tight chamber. I use the Galazan $400 bore micrometer which will measure a 12 gauge chamber. It will give you an exact measurement at all points of the chamber. I wish a $90 bore micrometer would measure a 12 gauge chamber, but, unfortunately, it won't.
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