The accepted way to measure the volume of empty brass cartridge cases is by the use of water.  One simply would fill the case to the mouth with water, and then weigh the volume of water in Grains.   

    For instance,

    32-40 Ballard case volume =  41 Grains
    32-20 WCF case volume    =  22 Grains

   Whereas;

    .357 Magnum case volume  =   27 Grain
    .357 Maximum case volume =  34 Grains

  For purposes of comparing the interior volume of the 32-40 Ballard case with that of the .32 Miller Short cartridge, one would need to be aware that the .32 Miller Short is a reduced length .357 Maximum case, which is factory length at 1.605 inches.  The Miller Short case on the other hand is 1.274 inches in length.  By doing a proportional adjustment, the above posted volume for the .357 Maximum may be reduced to coincide with the shorter length of the .32 Miller Short.

   For Reference, see:

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    Creedmoormatch