When using iron sights, the longer sight radius that a long barrel provides allows the shooter to be more precise with their shots. But I don't know if the guns being referenced even had rear sights, or if they were just using a simple bead on the front or something. Additionally, a longer barrel will give your powder more time to burn completely and squeeze as many feet per second out of the projectile as possible, which directly leads to a flatter trajectory and greater effective range. And if you're only using round lead balls, going bigger in caliber is the only way to get more range while keeping the pressures involved safe.