I developed those Stoney Point tools back "in the day". First the OAL Gauge and Modified Cases, then the Bullet Comparator and inserts....and then the Headspace Gauges....and finally the Rim-thickenss gauges for rimfire ammo.. Now......those products are all made by Horandy whom is doing a great job from what I understand....but made much the way Stoney Point made 'em. Pretty simple, and straight forward concepts and easy to use for those seeking improved loads and accuracy.
The Headspace Gauges came about when I realized that most of the bottleneck cases made at that time, had standard SAMMI "datum lines" dimensions called out on the shoulders of those cases that were meant to measure chambers and cases. Only a few dimensions actually.. At this point in time....most of these dimensions and proper sizing were pretty much left to the imagination....as nobody was able to measure such things as a datum line (some arbitrary point on the shoulder) to the case head. This gauge resolved that point.
So......I set out to do this via a simple set of gauges. Are they perfect for anyone and everybody? I suppose not. But they can and do separate differences in fired and unfired cases from one another, in order to set up dies, and a host of other things. Are they the "end all" in case size determinations? Maybe....maybe not. Some of these things are quite elusive. So many variables exist and some were not able to be even discussed until this (or other) gauges came to be.
Still, at the end of the day, these gauges will give you good ideas on case dimentional changes and the ability to re-size your cases to property fit the chambers of your guns. I'm pretty proud of what these products are able to provide for the avid reloader, and I think this product provides more insight than what was / is available in the past. Some like smoke and mirrors......some like facts and data. I prefer the latter.