Great thread. Lots of information here. Some primer flattening here and there is no problem. If it was combined with sticky extraction or sticky bolt lift... that's a problem. Additionally, you may have a bit or catering as an additional sign.

If the brass was on its first or second firing, it it still stretching ever-so-much to fill the chamber in all directions. That extra little bit of space allows the primer to back out just a hair as pressure builds and it expands a bit since the side walls of the primer are not supported by the primer pocket..., then the casing moves back under the rising pressure curve and pushes back onto the primer.

I personally think brass becomes the most consistent in a rifle after it has been fired in that rifle 3-4 times and only being neck sized each time.

As an aside..... When I shot .308, I shot just the 175g Matchking out to 1200 yards. I was pushing mine right about 2700 fps out of a 26" barrel and just barely off the lands (no more than .010").   It hammered 2MOA steel at 1000 all day long.  Different cartridge aside, the velocity was was kept me consistent at that distance.

I know its more money, but for long range shooting, a chronograph is a nearly indispensable tool. It allows you to collect the necessary data (standard deviation and extreme spread) to really make to most of Long distance.

I use a Magnetospeed Sporter which is the cheaper version at $179.  Yes, it changes your POI while it's on because it's changing the barrel harmonics. But the POI shift is consistent, I.e. my POI shifts up 5 moa on my Tikka T3x with its "lite" contour barrel. The group size is the same with the MagSpeed on or off the barrel.

So, whenever I shoot a load development series, I don't "zero" the rifle with the thing on.  It gets zero'd once I pick the load after the mad scientist portion is done.