Polygonal rifling has neither lands nor grooves. Look at any hex (hexagonal, six-sided) nut, ignore the threaded hole, and you have in your hand an approximate solid model of the hole (bore) in a cross-section of a Whitworth barrel from about 1860 AD. The "corners" of the Whitworth rifling were rounded, not sharp like the corners of the nut. Otherwise, the nut'll give you an idea of what six-sided polygonal rifling looked like -- six straight sides, six arcs connecting them.
I have no idea how much easier or harder it is to rifle a barrel this way -- or how much better or worse than land-and-groove rifling this kind of rifling may be. It obviously deforms round bullets and produces UNround projectiles -- whether they fly with more or less air turbulence around them, I can't guess and certainly don't know. I assume that if this kind of rifling were in major ways superior to land-and-groove rifling, it'd be the norm in custom barrels at least, if not in production barrels. My guess is that its main attraction lies in its novelty.
