So headspace is pretty much the length of your chamber. Too short and your bolt won’t close because the cartridge is holding it back, too long and there will be too much room for the round so when it fires the brass expands out and if it’s got enough room the tiny explosion could be catastrophic to the gun or you. Also it’ll put your factory ammo set too far back from the throat and rifling. Imagine a semi hitting an over pass that’s too low and all that force pushing it can only go back or out now. No bueno. Hence why tolerance for headspace is on 0.004 of an inch from bolt closing on the round to the chamber being too long.
Now everything has tolerances, headspace is .004” but most other lengths on the barrel threads are +-.003”. So that’s the length of the threads, the relief cut, the angle, etc. So one thread might be cut to -.097 and the other -.103 and both are still in tolerance but headspace on one would tighten down .006 more on one and be .006 too loose on the other.
All the measurements to determine the correct cut lengths for a barrel come off the action and the bolt so if you get a upgraded bolt from PTG it might be thicker which means it’ll push the cartridge in more and then you’re too tight on headspace. Same with switching Remington 700 lugs, my buddies swapped and suddenly his gun blew headspace checks because the lug was thicker.
I’ve started just slowly buying more headspace gauges, I like PTG the most, but if you want a cheap rental then 4D reamers carries about everything and does headspace rentals.
And yes every time you take a barrel off and put it back on it tightens down more. And the force used tightens it down more too. We do barrels to 35 ft-lbs and one guy took his to 60/70 ft-lbs just so he had a perfect tight chamber. Though if he just listened to Ackley man and fireformed his brass and then used only neck sizing dies and reloaded he wouldn’t need that.
