Daryl

I have used once fired brass from GECO, DAG and S&B as those are cheap available.
The 9,3x57 cartridge is listed with dimensions in the “Dritte Verordnung zum Waffengesetz” on December the 22, 1976.
If you compare the distance from the base to where the shoulder starts which is called L1 you have the following difference
8x57IS, L1 = 46,20mm
9,3x57, L1 = 45,86mm
So at least by this data the 8x57 should be too long in the shoulder area.

Let us compare the max outer diameter at the neck at the mouth called H2
9,3x57, H2 = 9,90mm and a listed bullet diameter of 9,25mm you have a difference of 0,65mm which lead to a neck wall thickness of 0,325mm.
8x57IS, H2 = 9,08mm and a listed bullet diameter of 8,22mm you have a difference of 0,86mm
which lead to a neck wall thickness of 0,43mm.

Even if some of the neck thickness is lost during the neck up process, it is no wonder that formed 8x57 brass could be too thick in the neck area.

Robert