My 20 bore English styled smoothbore, made by A. Hunkeler has about .005" choke- a weak Imp Cyl. Loading round balls is exceedingly easy, once they are short started,yet it doesn't blow patches & is fairly accurate. So far, inlearning to shoot this gun, I've not done better than 4" at 50 yards off the bench with round balls, nor have I treid shot in it yet, but expect it to throw very good patterns.
DP is spot-on about the lower pressures with BP shot loads, in comparrison to modern shotshells typical 11,500PSI standard- 3 1/2" mags, or 2 3/4" trap loads - all are loaded to the same pressure. Specialty ammo might be laoded to lower pressures, but it isn't common. With black powder loads, one must be careful about wad pressure to have consistancy, shot to shot. 50 pounds pressure on the wads was the old standard and works well in either shotshells loaded with BP or with shot in a ML. If you handload smokeless powders, you can reduce pressures and improve patterns to match those that work well in black powder, but handloading is usually necessary.
I believe the Amercian Roper developed a screw-on choke around 1850.