The steel case rounds selected were the three most rusted appearing cartridges I have, After popping the projectiles out using a [hammer type] kinetic puller I was pleasantly surprised to find no visible signs of rust inside the cases, the powder was clean and new looking, the insides of the cases looked as if they came off the production line only yesterday, the flash holes in the case head were clean and sharp and the projectile including the base of the projectile were “as new” .
I know many factors in the 65 or so years these cartridges have been around have contributed to the state they are in today, but my conclusion is to clean only the outside of the other steel cased 9mm PARRA pre 1945 German rounds in my collection,
Perhaps the 7.92x57 rounds had a harder life, or the steel is of a different structure, from different mills, or loaded with a more corrosive powder, I simply do not know, BUT at least one of each of the steel cased rusty cartridges of German pre 1945 7.92 x 57 in my collection will be dis-assembled and cleaned and “preserved”.
Just a footnote, I would be very hesitant in using these 7.92 x 57 cartridges in a firearm you had any respect for!!!