Brass can be damaged on the very first firing. It can not be fixed.
Out of curiosity, why couldn't the OP simply size to whatever dimension just allows the bolt to close,
and quit worrying about that aspect of the problem.
If headspace is excessive on the very first firing of new brass or factory ammo, the case may fail if there is more then .010" slop in the fit. I feel it takes at least .014" or more for the brass to fail on the first firing. It may take more than 1 firing, this is why factory ammo, that is almost always under size, does not fail with 1 firing. But the brass is already damaged on that 1 firing. Some old military rifles with lots of excessive headspace will get case head separation every time with new factory ammo.