I have shot ML for a long time and have tried many variables in shot and powders. No one has mentioned barrel length in regards to powder burn. I shoot a 58 cal flintlock with a 42 inch barrel. I shoot 2F powder and feel it gets a complete powder burn in a 42 inch barrel length. Shooting a Hawken style gun with a short barrel, I would shoot 3F powder. Partly because of the short barrel but also the 3F powder would fill the side breech better, being a finer powder. In a flintlock I add a flash pan powder that is a 4F or finer. Actually I take my 2F powder and crush it with my wooden powder horn stopper and use it in the pan. Some of it is a super fine powder that is dust. When the flint strikes and the sparks drop, the frizzen raises which will pull some of the dust up into the air. The spark is igniting the dust first then the remainder. It is a fast ignition given all other lock timing is correct. All powder is coated to make it less hazardous from friction, therefore even purchased 4F is not as good as crushing your own. Just my take and experiences with black powder. Hope it helps or gives another perspective.
