Wow, if you're going to open a can of worms it might as well be a big one! The tools you mention are different; the case gauge measures the cartridge or case and the Stoney Point/Hornady comparator measures the chamber. Some loaders use both types, some use neither but most serious rifle shooters use one or both types of headspace measuring tools. There is more than one way to measure a chamber and more than one way to use the measurements you gain. I only know what works for me in my rifles, AR's are a bit different so will let folks with more knowledge of them get into the details.
Life Member NRA, TSRA
Smokeless powder is a passing fad! -Steve Garbe
I hate rude behavior in a man. I won't tolerate it. -Woodrow F. Call Lonesome Dove
My favorite recipes start out with a handful of used wheelweights.