A couple of days ago me and a friend of mine (legal gun trader) were wondering what the heck is this! He possess a handful of these cartridges - with magnetic bullet (GMCS) jackets, and the tips of the two of the bullets were painted silver (or white, as far as I remember). Now we have an almost clear answer - except the exact designation of the caliber - 7.92x?
We had take a bunch of measurements and the length of the cases was very unconsistent.
I have his permission to use the pictures, so here we go.
Here are the measures of one of those cartridges, compared with 6,5x54 Greek Mannlicher-Shoenauer, the base and the cartridge as a whole.
Take care and have fun!