I tried this 8x57 "Russian" stuff. I put that in quotes, as it's actually German manufacture that was sold or given to Romania, and than later "captured" by the Russians. Recently it was rebuilt in Russia for commercial sale with non corrosive primers.
I've chronographed it, and broke a bunch down and reloaded the components. I've posted about this every time is comes up, but the short and sweet is.... It's a combination of soft primers, and maybe casings that have been over annealed. They are not actually overpressure, they do show some pretty concearning signs. I know they are fine, because I've reloaded them in modern cases that had less volume, gained an extra 100fps, and were fine.