The late Oscar showed how he added a spring to the back side of the rod to hold it against the barrel better. I lost my old file of his post. Perhaps other here still have one to share. I find that vertical works best for me and hang the barrels from the ceiling and move the gauge around. The spring holds the rod fairly firmly against the inside of the barrel which makes measurements easier to do.
If you have any doubt about the results it is very hard to duplicate errors. If you get the same results repeatedly then you are most likely right. Try measuring with a set of calipers and make the calipers slightly off by say .004-.006 oversize. Then adjust, repeat and see how hard it is to repeat the same measurement even when you are trying. I am not saying that you can not be wrong but to be wrong repeatedly is very hard even when you are trying.
