JTD, I finally got the time to really look into this tool this morning, and it looks like a great addition to a precision reloading bench. The term that was foreign to me was "datum" so I didn't quite get the function here. I have the Hornady LnL headspace comparator on my bench for many years already and I use it extensively for both headspace and cbto measurements. It has inserts for both headspace and bullet ogive diameters, so both tools do the same thing, but I do admit, if I didn't already have the LnL, the Forster one is certainly a nicer and more compact "made in USA" kit. Thank you for spurring me on to a little more knowledge in this great hobby.