“Rifled” slugs, also called Foster slugs after their inventor, Karl Foster, who came up with the idea in the 30s, aren’t really rifled. You can see the “rifling” on the unfired slug. On the fired slug, the vanes have been squashed flat, which is what is supposed to happen. Vanes do not make the slug spin. They may originally have been set at an angle in hopes of making the slug spin, or, more likely, in hopes of making hunters think they spin, but, what they really do is help the slug swage down through the choke.