That is because when the powder is compressed, Winchester compressed their powder between .17" to .21", only the powder closest to the mouth is compressed. The powder on down into the case and down at the primer are not compressed. Both John Kort and myself have dissected quite a few original cartridges to discover such applications. John described the powder as a "mixture" of FFg and FFFg of which he said was an original type of black "sporting" powder at one time.
I have pressure tested a few black powder cartridge loads and the results will surprise you. I know folks don't like clicking external links but I am not going to repost the data.
https://docs.google.com/spreadsheets...#gid=598633334
Bryan have you figured out whats the reason for the jump in pressure (and velocity) with those old cases? (semi balloon head - the last few on the chart) would think it must be something to do with better (quicker) ignition due to the primer pocket protruding into the base of the charge - but that is no more than a guess. ?
absolutely agree with you on compression - different effect down the charge - and longer skinnier the case the more the difference, say a 38/55 - bottleneck cases moreso different again - my 45/75 shouts this out.