Could be totally off course on this, but has always seemed to me any "Formula" for determining graulation of BP based on the dia of the bore is a hang-over from round ball days. Would seem that with elongated bullets this would go totally out the window & with shot loads as well. With a given brand of BP, granulation is essentially the only burn rate control we have. Higher load densities need slower burning powders. Any 28ga load heavier than 5/8oz or .410 load heavier than 3/8oz have a higher load density than a 12ga with 1 1/8oz. With modern powders 3/4oz & heavier 28 ga loads & all .410 loads call for slower powders than do standard 12ga loads. I see no reason it should be different for BP, though the total variation available is not so great with black.
