hlg,
You state "no headspace problems". Have you actually checked the headspace, then checked THAT against actual rim thickness? Factory brass is notoriously often under "spec". I'll bet the actual rim thickness is thin enough to give you those backed out primers. The case is holding tight enough not to stretch, and is giving the primers room to back out.
You might try lubing (lightly) one case, and firing it to see if something different happens. I'm guessing you will see a flattened looking primer - caused by the primer backing out, then being reseated when the case slides back to the breech face.
It looks like high pressure, but it isn't.
If that's what happens, then lube cases to fireform, then back off the sizer so the shoulder is just kissed. Shouldn't happen anymore. Hope this helps,,,,,,,,,,Bug.
