Originally Posted by Bart B
[...]Note that how the cartridge is supported makes a big difference in the amount of runout there is. Especially when there's no such thing as a perfectly round case body. Rimless bottleneck cases, when fired, center in the chamber's front end at its shoulder, not anywhere on the body. Any out of round condition of the body right behind the shoulder will be reflected in bullet runout if that point's one support location. It's better to support the front of the cartridge about mid point at its shoulder. It matters little where the back of the cartridge is supported.[...]
Perhaps Hornady has improved the quality of their guages since I bought mine? Mine has measurable slop. But, I've heard similar comments from others and have checked several at stores and at DSC. All the ones I've handled had slop in them.
Charlie's may be better than mine and it appears that his reads in tenths. Or, perhaps a typo?
Nonetheless, it all goes back to Bart's point. i.e. you'll get different readings depending on how you hold the cartridge and where you measure
The Hornady method supporting the cartrdge between the case head and bullet tip is flawed IMHO.