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I always understood 9MM, in the case of the 9X19, Parabellum or Luger, to have a nominal bullet diameter of 0.355 - 0.356". This compares to the nominal .38 Special or .357 Magnum bullet diameter of 0.357 - 0.358". So to me the difference has always been between 0.001" and 0.003". For qualification caliber (chambering) requirements, such a small difference can easily be ignored, particularly when the energy of the loadings in 9MM are sometimes greater than those in .38 Special, not to mention earlier .38 S&W loads which would technically meet the minimum caliber requirements.