Stan, the flame front of Black Powder is sub sonic, while modern propellants (in the chamber) are supersonic. Modern powders are almost entirely consumed before the shot leaves the cartridge. With black powder the lad has moved before burning is complete, so it is much harder to impossible to get the same acceleration with black powder.
Fortunately for BP shooters, the difference between velocities at say 25 yards of 1300 fps and 1150 fps shells is somewhere in the vicinity of 50 fps, so it's less important than it appears on the box top.
