I set my headspace measurement to 1.462 on my 223 brass. On my AR it expands to 1.466". The other other day I was shooting subsonic (reduced loads) in this brass (trail boss). Measuring the headspace on these loads showed that the brass headspace had decreased to 1.461.
I double checked and re-zeroed my caliper, even made a small dent on the brass to see if the powder expanded the lower part of the cartridge (it did). despite all this the headspace still seemed to of decreased.
I'm not sure whats going on...
Gas guns are designed to unlock when there is still a residual pressure in the barrel. Chin calls this the residual blowback effect and I believe it is there to add additional time for useful gas pressure.
See the inset, that is the period during which unlock occurs in the 308 .
Of course the gas pressure cannot be so high that the case sticks too tightly to the chamber or the rim will be pulled off or the case will be ripped in half.
Roller bolts open so early in the pressure curve that the only way they can function without ripping the case in half is with gas lubrication. The upper 2/3 rds of the chamber is fluted so barrel pressure can float the upper 2/3rds of the case. The bottom 1/3 serves as the case seal.
But given that there is some gas pressure at unlock and extraction in your mechanism, and you don't have chamber flutes, it makes sense that the thinnest part of the case is going to be sticking to the front of the chamber, and that the case is going to get stretched during extraction.
And I believe that is what you are measuring with the standard pressure cases.
I started leaving the lube on my M1a cases, or rubbing loaded cases with Johnson paste wax to reduce case stretch. I don't have case head separations with appropriate powders and charges. The front end of my cases don't stick to the chamber during extraction and my cases do not get stretched.
I have run lubed cases in the AR but I don’t recall checking headspace of the fired case, though I know I have. I suspect they did not grow either.
Your light loads, I suspect unlock occurred when the residual breech pressure was too low to stretch the cases.