Conducting some tests with burning efficiency of black powder cartridge guns. Normally I load the 38/55 with Swiss No2 (3F) which burns very cleanly as determined by wiping the barrel after every shot and noting the fowling on the patch.
Having been given a few part tins of various powder, I thought I would burn some up, first 'Elephant FG' 50gns in the 38/55 using my tried Paul Jones 375gn custom lathe bored bullet was tried and I found that it burned exceptionly clean with hardly any residue on the patch. Switching to the same load but with a 220gn Westcasting there was approx 3 times as much fouling and even some unburnt granules.
Thinking about it, the much larger bullet doesn't move as quickly as the smaller one, creating a much higher pressure peak in the cartridge case as the powder burns. Thinking back to the high school physics P1V1T1=P2V2T2, if the volume doesn't change (i.e. the heavy bullet doesn't move and therefore change the expedential volume in which it burns, the temperature must get hotter within the chamber, thereby burning the powder more efficently.
Today I switched to the 50/70 firstly using a Lyman 425gn bullet followed by a CBE 520gn one. The same thing happened, the heavier pill load burnt cleaner than the light one (but not a dramatic difference as in the 38/55
So have the 'experts' got it wrong? It is normally recommended that the bigger the bore the courser grade of powder should be used.
The next set of tests is to try some loads of 3F and 4F in the same rifles, and in my muzzleloaders which range from 0.446" to 0.577". There are a lot of big heavy boolits for those also!