The central flash hole makes sense just from an ignition standpoint.
With the pellet being molded and compressed, it would likely be difficult, if not impossible, for the flash to propagate through the powder.
Given that black powder has a distinct, and fairly slow, burning rate, what you'd get would be a rocket motor. Not particularly suitable for propeling a bullet.
At the tail end of the blackpowder era came brown, or cocoa, powder, which used incompletely charred charcoal.
This slowed down the burning rate considerably, but it allowed for greater velocities (slower burning equals longer push on the projectile).
Because it was more difficult to ignite, manufacturers wet molded it and often included piercings to assist in ignition and in burning.
European Prismatic Powder and American Hexagonal Powder were both shaped like hexagons of varying lengths depending on the size of the gun in which it was used. Prismatic powder had multiple piercings while Hexagonal (developed by Du Pont) had a single central piercing, which made it look like a nut.
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