Good point savageT, The barrels on my cap & ball revolvers appear (key word) to be straight , untapered stock, so I was planning to use a very small machinists square. You are correct, how could 60 year old eyes see a ten thousanth ? Luckily, all of my revolvers shoot better than I can, so I won't worry about it, but if one was inaccurate, it would be time to write a check out to a person who has the right tools ! Cowpox