Before Jon Hosford built his tool, he used to show others how to use the Manson gauge in a horizontal fashion. There are several threads in the archive on his technique, I believe. It involves suspending the dial end of the gauge with a cord in order to let gravity center the other end at the lowest portion of the barrel. If you try to hold the dial end with your hand, you put unwanted forces on the probe that move it up and down the sides of the bore.
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