Colonial Willamsburg filmed the flintlock's ignition. They were startled to learn that first sparks actually jumped out of the pan. They saw fewer jumping with the larger grain powder and surmised that the larger grains captured the sparks; thereby causing ignition.
Remember that the soldiers of the 18th Century did not carry priming horns. They primed from their cartridges.
Vigilantibus et non dormientibus jura subveniunt. Molon Labe!