Yikes. I didnt realize PPU had brass that had corrosive components. But i also dont know much about PPU other than they produce almost any caliber you can imagine.
I believe it was the Bulgarian surplus stuff that did this quit often in 7.62X54R.
That looks like the brass might have been in a leather pouch/holder since the level of corrosion is all the same height.
As to PPU I don’t believe any of their commercial stuff has corrosive primers. Now Yugo military ammo it is corrosive.
Countries that used brass case is/was:
Yugoslavia
Albania
Bulgaria
Soviet Union/pre-Revolution Czarist forces
Germany WW I in captured Mosin
Finland
Egyptian
US companies (WW 1 contract and after
up to todays loading by Remington)
The Mosin rifles were originally developed with brass cased ammo and went to steel under Soviet “upgrading”.
This situation has me wondering about the chambering of the rifle, if it was one of the poorly made conversions to 8mm or Bannermann style 30-06, or just weakened brass for to the corrosion/erosion of the brass?