There might be something like that going on as well,never tested it.
I know one other way to get unburnt powder specs is when the primer(usually a mag primer) starts to move or unseats the bullet before the powder gets a chance to ignite well, so because the bullet is moving and creating more volume for the powder,it never builds up enough pressure to burn efficiently.
I did test that with a couple loads using small mag primer in 45ACP, changed to standard and the powder cleaned up plus they shot better groups and felt stronger.
That my theory and I'm sticking to it
