I'm one of those tragics who collects snippets of mainly useless information unless I play Trivial Pursuits, which I don't.
When a container is vibrated, the contents rise up thru the centre & circulate back down round its edges. They don't sit still. Now that's all fine when the items in it are all the same size, like a jar of filberts. But if it's a mix of filberts, peanuts & walnuts, for example & it's vibrated for a time, you'll end up with peanuts at the bottom & the larger nuts on top & the filberts round the mid point, because the peanuts can work down thru the gaps between the bigger ones & the filberts past the walnuts.
Now it seems to me that a cartridge case of powder is about the same as a jar is. So if you plan to mix powders of different granulation, make sure you never subject the loaded cases to vibration, like when you drive a while to the rifle range, or it will unmix itself with the smaller granules down the bottom & the biggies up near the neck. Hereabouts that would likely mean that the fast burning powder would ignite first & the slower powder later - not necessarily the most desirable outcome.