I haven't loaded much BP beyond my 1858 Remington .44 Ball and Cap.  However, I've been told multiple times that Cream of Wheat is just the thing to take up the space when loading BP cartridges when using reduced loads to keep everything nice and compressed.  I'm going to be loading 45/70 sometime in the near future... and since it'll be for an original 1873 Carbine (405 gr and 55 grains), I don't want to FUBAR anything!