You are absolutely correct. Once a chamber is cut with a reamer, the headspace is set. All a shooter can do is measure his/her her brass to set how much brass flow they want with each shot.In my mind, headspace is gauging the length of the chamber from datum to bolt face. Thanks to the internet "headspace" now means 2 different things, one of which is not correct. Cartridge fit is not headspace. Only the distance from datum to bolt face is. Brass is not made to the same tolerances that hardened steel gauges are. Brass is malleable, it can be shaped easily. Upon firing the brass 'swells' to me the chamber walls and when pressure drops, it contracts. Any material that can move that much is not a "gauge".
What Bill says makes sense.. By jamming the bullet, when a 'long' chamber exists, the brass stretches at the shoulder end, not the head. Reload from those fireformed with the desired shoulder 'bump'. All this must be taken into reason, of course. If the chamber was groosly long, it'd need fixed to be safe.And I would fire form some brass with bullets seated out.
I think 'crystalgaleguy' is taking about factory take-offs. In which case, there not much use in putting too many $$$ into one just to say the headspace is GO + .002.Too much headspace can and is most often the cause of light primer strikes. I try for .002 max using "scotch" tape on the head of the headspace gauge is my no-go gauge. Tape is approximately .002 thick.
Factory take offs were not necessarily chambered correctly the first time at the factory. Production pressure has literally forced too many shortcuts to doing things the correct way. I've checked go-gauge protrusion on many take offs. Some are ok while some chambers appear too deep. DrI think 'crystalgaleguy' is taking about factory take-offs. In which case, there not much use in putting too many $$$ into one just to say the headspace is GO + .002.