I played with one of these about 10 years ago. After spending a lot of time with measuring ammo in the same lot number, I found that the case heads were not flat and if you oriented the hull in the holder you would get a different measurement. I decided that measuring the rimfire thickness means nothing unless you orient them into your chamber the same way that you measured them. This was too mind boggling for me to keep with and decided it was not worth the effort. Is easier to just spend a little more and buy better ammo, IMO. Don