“... with flashpan powder being of a finer grainage than the barrel powder - making it a two-flask operation.”
By the 1770s loose powder poured from the flask was not used in combat.
To the greatest extent possible, troops were supplied with pre-loaded cartridges. The cartridge was torn open, pan primed first, then barrel loaded.
This did cause a less-than-efficient use of powder granulation: the main charge worked better with a coarse granulation, while the pan charge worked better with fine granulation. A balance was struck - cartridges were loaded with intermediate granulations.
Schurmann wrote:
A lot.
But, hell, what do I know? I owned a nice Dixie Gun Works “finish-at-home” Pennsylvania flintlock rifle, with the Roman stock, a 36-inch browned barrel, and all the German silver work to polish, the lock to fit, and work, chambered in 45-caliber, having used a 435-ball and a 015 patch, over 65 grains of FFFg powder (Pyrodex was not there yet), a flash pan full of FFFFg powder, ignited by a nice small English flint, for over 10 years. And yes, it was ‘blooded’.