If I remember correctly, the 9 x 21 is loaded to the same OAL as the 9 mm Parabellum, has the same boiler room and the same ballistics, and is intended to sidestep the laws of various countries that ban military chamberings. The 9 x 23 was a US wildcat or proprietary cartridge designed to make major for competitive shooting. It has a little more OAL length and a little more boiler room.
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