Colonial Williamsburg used high speed film to capture the flintlock's ignition. They saw the sparks rain down into the pan and then leap out. They surmised that until a larger grain or collection of grains capture a spark and holds it long enough for ignition. We're talking micro-seconds.
Military cartridges only had one size grain and it wasn't until the Napoleonic Wars that military riflemen had priming flasks with finer powder. Up to then, soldiers primed from the cartridge.
Vigilantibus et non dormientibus jura subveniunt. Molon Labe!