In the beginning there was the 7.65 Borchardt cartridge. Loaded up for the 7.63 Mauser with no significant change of diminsion. Then shortend to form the 7.65 mm Parabellum.
DWM's first attempt at a 9 mm Luger cartridge was the 7.65 mm Parabellum necked up to 9 mm. This resulted in a bottle necked case with a shoulder either to too small for reliable headspacing or just too small to bother with. The next attempt was a taperd case and the 9 mm Parabellum was born.
My point is that if you bore out a 7.65 Parabellum or a 7.63 Mauser chamber, there will be an area where the should was that will be larger in diameter than a 9 mm chamber, resulting in an odd shaped chamber with a "ring bulge" in it. The shoulder on the bottle necked cartridges is larger in diameter than the corresponding point on the 9 mm Parabellum case. Seems to me that to get a proper chamber you would be forced to sleeve it.
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