I think there'll be a bigger difference in down-range bullet speed and point of impact (change in zeros) across atmospheric conditions the rifle and its ammo's shot in than the difference between new and fire formed (as well as resized) brass observed in a single comparison session. Note also that BDC turrets are only very accurate at longer ranges with one load in one set of atmospheric conditions. Using that load in different elevations, temperatures and humidities will easily need up to 6 MOA changes in sight elevation settings at long range depending on the cartridge used.
There may well be a few fps difference between muzzle velocities with the same case when first fired when new and fired after it's been resized with the same load in the same conditions. If the difference is 10 fps, that'll mean about a 2 to 3 inch change in vertical shot placement at 1000 yards for several 30 caliber magnums as computed by ballistic software. If that's cancelled out by where the bullet leaves the muzzle in its upswing from recoil (faster ones leaving sooner, slower ones later), it may well strike the same place as a bullet leaving 10 fps different.
I don't think a few thousandths spread in head clearance (space between bolt face and case head when the round's fired) across loaded rounds has any significant effect on accuracy. If it's more than 3/1000ths inch, it can lead to case head separation.
