This question already has an answer here:
I think this question is already answered anywhere, but I couldn't find a solution with my search terms :/ . So I got this model and I only want to smooth out the cylinder parts, but not the extruded edges.
So here is my original model, I think it's kinda clear what should be smoothed out, it is supposed to become a rifle scope.
This happens with the normal smooth button, but I already tried it with a Bevel modifier and subdivison surface aswell.
Solution: Edge Split Modifier with Smooth: