Black Powder does not GO OFF all at once. Back in 1970 I built a .32 cal Squirrel Long rifle. It had a slow round ball twist. When loading it light for squirrel there would be a spot in the barrel which was extremely fouled, preventing loading another ball without cleaning. Increasing the charge moved it toward the muzzle, but it took a real heavy charge to move it all the way out the muzzle. Black powder was slower burning than available smokeless powders until the development of the "Progressive" powders used in the new high velocity/heavy shot charge shells which appeared in the 1920's.
