Going back to your post # 557779 on 11/4/19, there is an excessive expansion of the neck of the fired case that is easily visible without measuring. Is this really a 6.5 m/m?
Then, assuming your Sellier & Beloit ammo was new unfired brass, there had to be a considerable difference between the case dimensions and the chamber for the brass to split as it did, and have a partial head separation, instead of just fire forming.
I'm thinking your light primer strikes were possibly caused by the firing pin pushing the case off of the bolt face because the shoulder was several thousandths back from contacting the chamber. This looks a lot like an excessive headspace situation, and the recommendation to measure with headspace gauges seems like a great idea. That might explain the strange deformation at the case-shoulder juncture, and the partial head separation in new brass. Was the primer very flattened, which could indicate that the case head was unsupported by the bolt face at the instant of firing?
I think I'd start with headspace and bore measurements, followed by another chamber cast, and careful measurements of the Cerrosafe at the correct time to get exact dimensions. It wouldn't hurt to find a soft lead ball slightly over-size to slug the bore, and get precise land and groove dimensions. It seems strange to not see even a trace of rifling on your chamber cast.
It is unusual that a chamber would be polished that far out of spec unless it was severely pitted. I don't see anything in 6.5 cal that has dimensions very similar to what you measured. This could be some wildcat round. Or you might end up setting the barrel back a couple threads and rechambering... or rebarreling. I'd sure be wearing good shooting glasses, or better yet, firing this rifle tied to a tire with a long string on the trigger, until you get this sorted out. I once spent about a half hour totally blind in both eyes after having a complete head separation with a VZ-24 Mauser rechambered to .22-250. That gets your attention, and it isn't worth taking such risks.
