"The barrel length does not have a lot do to with powder burnup."
One needs to factor in the pressure at which a particular cartridge is designed to work at.
A short-barreled .357 Magnum loaded with 296 will leave behind a lot more unburned powder than one with a longer barrel. The longer barrel allows for a longer dwell of higher pressure, giving a better overall burn, whereas with the short barrel the pressure drops off much faster, leading to more unburned powder.
In a .38 Special, though, a slow burning powder may well never reach an optimal peak burning pressure, and so will tend to be a lot dirtier and a lot more wasteful of powder no matter what barrel length you use.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
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