There is a practical max velocity limit, since black powder velocity is related to barrel length more so than with smokeless powders. You could, for instance, have a barrel 20 feet long that could give extremely high velocities, but it would not be practical for anything, not even target shooting.
Unfortunately, increasing your standard black powder load to the maximum recommended load does not produce a drastic velocity difference like you get when increasing a smokeless powder charge. The pressures involved increase dramatically, but the velocity does not. The difference may only be 200 to 300 FPS, which in practical terms, is nothing. You can have that much variance - or more - between "identically loaded" shots! If you want 2,000 FPS, you are going to have to go with a cartridge gun, shooting smokeless loads.
All statements below are based on 30 years personal experience, using modern made conventional side-lock rifles, and do not apply to original antique arms, original or replica military muskets or rifles, or in-line rifles. Use of the load data below is at your own risk. Always check and follow the manufacturer's recommendations for recommended loads and charges. Most of the manufacturers I am familiar with, provide load data using FFFg or Volume Equivalent Pyrodex loads.
In my 30 or so years of using MODERN MADE conventional sidelock black powder rifles, I have found that when using a patched round ball, NOT A CONICAL you can safely use a load of 1 to 1.5 grains FFFg powder per caliber. IE a .45 caliber can use a load of 45 to 70 grains FFFg or equivalent Pyrodex. If you want a heavier charge than 1.5 grains per caliber, USING A PATCHED ROUND BALL projectile, you need to go to a coarser powder, either FFg or preferably Fg or Pyrodex "CTG" to avoid potentionally dangerous high pressures. Using Fg or CTG, I have found that up to 2.5 grains per caliber can be used before barrel pressures become a problem
As for "max range" in the U.S. Civil War, some Confederate snipers had the British WHITWORTH Rifle. This was a .45 caliber rifle, with an OCTAGON bore, and used a long (over 1.5 inches) bullet. This rifle was accurate on a man size target out to 800 to 1,000 yards! The .45-70 "Government" which began life in 1873 as a black powder cartridge, while today is considered a "short range" cartridge, was originally praised for its long range accuracy - out to 1,000 yards in competition shooting matches. Modern factory .45-70 cartridges, by the way, (other than those by Garrett Cartridges, of course,) are loaded to black powder trap-door rifle pressures due to the large number of original and replica Trap Door Springfield rifles still being shot. For modern arms suich as the Marlin and Winchester lever guns, bolt actions, Ruger No. 1 and No. 4, the factory loads are under loaded.