I assume what you really want is to know is the practical effects of how headspace affects what you should do when you size your reloaded cases rather than the techical stuff you've found in books? Okay, but forget what you "know" for a few minutes. Otherwise you will remain confused. I don't often write web essays but we ex-GIs got to stick together!
Where most of us 'experts' (and books) get new guys more confused than helped is by jumping directly into how and where headspace is measured for different types of cases; fergit it. None of that stuff is relivant to what you need to know to properly reload. Actually, the
applied info on headspace is actually pretty simple. Technicially defining what the books say is better left in the books because it only matters to geeks like me.
I'm assuming you do know that headspace, as such, is in the chamber. That's set/machined within a specified tolerance range during manufactor and we can largely ignore it. However, it's obvious our reloaded cartridges have to snuggly fit in our chamber. The 'head' of our cases must be controlled in the chamber or things start messing up - either we can't close the action or cases stretch too much for safety. We need to control the cartridge so it fits the chamber in such a way that the bolt can be closed and the cartridge not be too loose/sloppy a fit. That's really all we reloaders need do or understand. Alright?
Cases expand in diameter when fired. Resizing squeezes the diameter in and, just like squeezing a roll of PlayDough in our hand, the case gets longer as the die squeezes. That squeeze pushes the shoulder forward, eventually making it too long to chamber
unless we push the shoulder back to near where it was before we sized. Moving shoulders back maybe 3 thou for an auto loader or 0 to 1 thou for a bolt gun is plenty of set back to maintain all the "headspace" chamber fit we need.
Bottom line, all we need to do is
set our sizer die to do is restore the sized shoulders to match the fired shoulder location so there will be no slop - no 'headspace' - to worrry about. We can do that by carefully adjusting the sizing die down - (correctly, more below) - and test fitting the sized cases in the chamber as we go. Precision measuring tools like Hornadys case headspace tools on a dial caliper or RCBS' Precision Case Mic make it easy.
Many times I read well intended folks saying to screw your sizer down to touch the shell holder plus "X" more turns. Nonsense! That 'formula' assumes all rifles, dies, shell holders, presses and cases are the same; they are not. Yeah, it makes ammo that will loosely chamber and fire but it rarely makes ammo that actually fits
our rifle! I also commonly see suggestions to get a Wilson type "drop in" cartridge/case gage but that's also nonsense, it makes finished ammo that fit and fire in any rifle ever made for that cartridge but there is nothing custom fitted about it. And I VERY OFTEN see BAD suggestions to
turn your sizer down in 'small' increments like 1/8 or 1/4 turn; NO! Full min. to max. headspace, from head to shoulder, in most bottle neck cartridges is only 6 thou. Even a 1/4 die turn potentially moves the shoulder about 18 thou and that's fully three times the total tolerance range!
A 1/16 turn moves the die about 4.5 thou and that's over two thirds of what we should aim for, all meaning small die steps should indeed be small and knowing that helps explain why using a precision case gage of some sort can be important.
Note that I've not mentioned how to measure anything from a rim or belt or certain kinds of specifications like shoulder datum lines. All that's interesting trivia for some people but it has NO
practical meaning to a reloade; just work off the shoulders and make them fit YOUR chamber and all will be well at your range.
Also note that I've deliberately not addressed any straight wall (auto pistol) cases that 'headspace' off the case mouth but that's because other than trimming the occasional over long cases (rare) there's nothing we need do about them.
Good luck!
