I can only answer in a vague sort of way.
There is a maximum velocity and apparently it depends on the resistance of the bullet in the bore. Apparently, the more resistance, the faster the round for a given weight of charge.
Case #1: Lead bullets seem to peak out around 1350 fps. There are examples that go higher with very much lighter bullets and finer granulations of powder. This was one of the reasons that the black powder guns got bigger and bigger in bore size--so that the velocity limit could push more lead.
Case#2: The .303 British was originally a black powder cartridge with a jacketed bullet. The original loading was about 1800 fps and that was attributed to the greater resistance of the jacketed bullet.
Hope that helps some.