Also another factor making a very slight inaccuracy in your 2-rail sweep method is that end tangents of a helix are not pointing in the same flat direction as the arc - a helix has a slight incline to its tangent direction and during the 2 rail sweep the profiles will follow a kind of average tangent between the 2 rails and so that will basically angle the profiles a little bit as the sweep traverses its path.

Basically if you want to have the most accuracy if you want a shape that is supposed to be a fragment of a cylinder it will be the most accurate to actually construct it literally that way by starting with a cylinder and then cutting it.

- Michael