20 sided cylinder cut off 1/4 12 sided cylinder cut off 1/4 Small manual tweak to get the shape close to the bigger cylinder 8 sided cylinder cut off 1/4 even with manual tweak the curved shape will not be as perfect as a cylinder with correct amount of segments.
A couple things with the screw. Image below: 1) It seems like your fencing is asymmetrical. See red lines. Just model 1/2 or even 1/4 of the screw, duplicate and mirror geometry to avoid overcomplicating it 2) N-gons tend to cause shading errors, and you had one. Though this seems like it can be avoided by doing #1 3)…
@drakewho looks like I found the reason why you have that shading artifact. Try to add this support loop: What level of subdivision and number of Bevel segments do you want to have? Level 1 and 1 segment?