For some time now I am working on loads for a Mannlicher – Schoenauer M 1905 in 9x56 M-S

I never encountered the same problems with any 9x57 Mauser, rimless or rimmed, rifle barrel. As my barrel is of .357” groove diameter, shooting .358” bullets from reformed 8x57 brass should not be a problem but it was. A chamber cast revealed the culprit: As Daryl_S noted above, P.O.Ackley once experimented with oversized bullets in grossly undersized barrels. His findings: It’s not the undersized barrel that raises pressure, but the too tight chamber neck and throat dimensions that allow no case neck expansion to release the bullet. Well, the Austrians had found out the same thing 40 years earlier, in 1916, when they adapted tens of thousands of conquered Russian Moisin – Nagant rifles to use their own 8x50R Mannlicher cartridges by simply rechambering with a generous neck and forcing cone and shot their .324” bullets through the.313” groove barrels. But in 1905 the Steyr people apparently did not know this. The chamber cast revealed an undersize neck area with barely 9.68 mm, even a tad below the now CIP minimum of 9.7 mm = .382”. Maybe a worn chambering reamer? Comparing the CIP numbers for the 9x56 M-S and the 9x57 Mauser revealed the general problem. As both cartridges use the same 9.08 mm = .357” bullets, the chamber and case neck diameters differ considerably: Max cartridge neck diameter of the M-S is 9.65 mm = .380” vs. 9.83 mm =.387” of the Mauser. Minimum Chamber neck is 9.7 mm = .382” vs. 9.88 mm = .389”. So the 9x56 M-S chamber has much less leeway regarding variations in bullet or case neck thickness. My solution: I will open up the neck area of my chamber with a 9.9 mm = .390” reamer.