When a powder charge is completely in the breech it burns progressively and even though this burn is rapid, the whole charge does not ignite at the same time. If, however, fouling (as found in that barrel tested by H.P. White) causes the charge to be spread out along part of the bore, it can "flash ignite" and all ignite at the same time, causing a significant pressure spike. Our son is a reenactor on a F&I War group, he has been taught that after pouring the blank charge down the bore, to slap the breech area of his musket sharply in the area opposite the lock, to make certain that the powder charge is in the breech area.