A similar "discovery" was made by author Sherman Bell a few years ago now and was documented as an installment in his "Finding Out for Myself" series in "The Double Gun and Single Shot Journal". He was preparing cartridges for use in a test involving a variety of brands of BP. The discrepancy/difference in weight per unit of volume between loads using Swiss and loads using Goex or Kik were on the same scale as noted
by Herb.
Thanks, Pete. Swiss powder is what got me weighing the volume loads. Now that I have some more Swiss 2F, I'll eventually shoot a series with the Swiss powders similar to the above, as soon as I get done building a couple of Hawkens.