I'm a retired tool-maker, any measuring too you use is only as good as the user. snap gauges (telescope gauge) is as accurate as an inside mike. Its just how you use them. Once you pull it through, don't do it again. Measure it and you're done. On any inside measureing tool its a good idea to put some light oil on the contact ends. Dial bores will always leave lines on bearings. A little oil will help a lot. The set up on dial bore gauges is very critical . If you don't have a ring gauge you can put your mike lightly in a vise lock the barrel on your mike to the size you want have the mike in vertical position. With the solid end of your dial gauge down on the center of the mike anvil. lightly swivel the plunger around on the top anvil and set it at your smallest reading. If you have Jo blocks its much easier and most accurate. Hope this is of some help. Good luck