The general, original theory about why the original X-Bullets didn't always group well was that since rifle bores vary slightly in diameter the X's had to match the bore-size well. Otherwise they'd have to be "squeezed down" too much--or be too "loose" to group well.
Which is why the grooved shank of TSX's eventually solved the problem, since it allowed the bullets to work well in bores of slightly varying dimensions.
Which is why other monolithic bullets generally have circumferential grooves--even if only one.
