Hi
I have been reloading for the last six months
After performing OCW I have found my perfect load on my LR 308 DPMS 24"

Varget 43.2gn, lapua brass, WLR primers and 168 SMK
COAL 2.230 (base to ogive)
Headspace 1.619"
This gave me 3 shot group .297" @ 100 yards

However My question is about headspace
Here is my data;
Lapua new brass Headspace 1.615"
Lapua once fire headspace 1.621"
I bump shoulder .002" to bring it to down to 1.619"

I notice that if a chamber a dummy sized round(headspace 1.619") and then I ejected the ammo, the shoulder goes back to about avg 1.615"
should I bump back shoulder to 1.615" or leave it at 1.619"?

I'd shoot for 1.618"
New brass is usually going to measure shorter so it will function in anything.

Gas guns usually stretch the brass upon ejection because of existing chamber pressure.  I have a .308 and a .260 and both stretch anywhere from .002-.004.  One thing you might try,  is to disable the gas system to make it a single shot and then measure a fired piece of brass.  With that, I'd then bump the shoulder back .002-.003.

To disable the gas system, I'd loosen the gas block and move it straight forward about .25" or more to block the gas hole.

I'd guess the cartridge was being resized in-situ by its inertia/momentum while being chambered.  It's not a big change, so I don't think I'd worry too much about it.

If you want to test this theory, manually close the bolt on a round.  If it is unsized by this slow chambering, then inertial resizing is likely the cause.  If you are unable to get the bolt into battery, it is difficult to get it into battery or the round is resized by manuallty operating the bolt, then your measurements are erroneous and your brass is just too long for your chamber.

Does that help?

THANKS Trollslayer

THAT helps a lot..and acording to other fourms where i have been asking the same question I am going to set back sholder from fire form size down to .003"

THANKS

Thanks glorifiedG

That is the setting i am goint to settle for!!!!
base on this and other fourms im goint to shoot for .003" back.  With my fire form brass it will be 1.618"

THANKS