Any sign of head stamping on any cartridge in that load lot is a fortunate warning to back down and work from there. For that partial moon to appear, the brass has to have reached a temperature adequate to allow/cause the brass to flow as a liquid.
Hi,
One can get extrusion and swipe marks in the brass and not be necessarily related to dangerous pressures.
Even factory ammo might show this and could be more a condition of the gun and/or bolt than anything else.
For example if the ejector plunger hole has not been properly chamfered or the chamber doesn't provide all the support the brass needs like a polished chamber that is a mistake
or a combination of these two and other factors.
This results in a high impact (obviously) and things get imprinted in the brass but no other signs that might indicate dangerous pressure is present.
It is not unusual for brand new service ammo to show plunger marks specially auto-loaders.
Lake city ammo even slightly flattened that is found many times with service ammo.
Again if there is trouble the signs normally come in twos or threes at the time.
If one has blow by or extreme extrusions or even blown primers that is a big NO NO.
I had brand new Hornady factory match ammo blow the primers on me and we know that is a huge problem.
My point is that we need to look at the whole picture and including brass, primers and the gun too / cambering situation too.
Only the OP can get close and inspect with care taking everything into consideration. So far I have not seen anything crazy
but only him can know paying a lot of attention to the detail. Measuring the base of the brass with the micrometer is a great
way to see if in fact there is deformation at the base like I believe you are suggesting as a possibility. Bras will always give a few
thousands through its serviceable life but because the base is compromise normally the pockets go first or incipient head situation
if one is over stretching the brass.
That is why I always headspace my bolt guns with the brass produced with my dies and the ammo I normally shoot.
For those who do not have a choice on selecting head-space, one can adjust dies and neck size only too obviously.