Some folks have been measuring new, fired and resized case headspace before and after firing since the 1950's. They've been measuring that headspace with tools before and after firing such cases for that long, too. How else would they know what the difference is to tiny dimensional standards?
They've used shims between a die's lock ring and the press since then, too. Most common shim and die this was done with is probably the .38 Special dies when set about 1/10th inch higher in the press so .357 Mag. cases could be reloaded with the same die set. I've been using a set of washers doing this for years.
Problem with shimming bottleneck rifle case dies up is when standard shell holders are used. Unless each fired and cleaned case has exactly the same amount of lube on it, how far it goes up into the die will vary several thousandths. Presses spring different amounts depending on the friction between case and die as the ram pushes fired cases up into dies.
So, shell holders with different heights allows the reloader to use one that when the die barely touches it as the press handle cams over at the top of the ram's stroke, sized case headspace has a much, much smaller spread. And the shoulder's moved to where they want it. Sometimes folks get a spread of less than 1/1000th inch. across several dozen bottleneck cases so sized to reposition their shoulder back a tiny bit.