2-piper,
Of course, properly presented plug gauges give the best measurement, but they are expensive enough that fewer people have them than have small hole gages, which give a decent measurement. The plug gauges come in .001" steps, so even if properly presented can result in .0009" error. With regard to nominal vs actual bullet diameters, other examples include 38 Super, 38 Special, 38 S&W, 38 Long Colt, 38 WCF, and 38-55; not to mention all the nominal designations for 22 caliber rifles.
Mike

Last edited by Der Ami; 10/18/18 10:54 AM.