Two things about the Brownell's Manson gauge.
First, I suggest you make the "Robson Modification", courtesy of MarkII. (shown on the gauge Mark made) It's a piece of metal packing strap, not quite as wide as the rod and slightly bent. It is compressed when it goes down the tube, providing tension to hold the tube against the rod for an accurate measurement.
Second, the gauge's rods are thin enough to measure 20ga and up, so they tend to move a bit, so there is a rubber stiffner provided that goes on both of the rods at the same time. You have to move it and re-zero the gauge each time you make a measurement.
I have mine mounted vertically, on a half inch piece of wood, which brings the gauge out from the wall so it's easy to move the barrel sets up and down along the gauge.
