I always figured the grooves in the nose give the nose the ability to be slightly oversized for a firm, centered fit in the lands, while not jamming hard and being tough to extract. This would probably benefit auto-loaders like the M1 and M1A quite a bit without having to be so precise in the alloy and nose shape department in order to find alignment at the end of a loose, oblique jump. The other advantage that jumps out at me is that the reduced surface area and ease of manually engraving the nose will work in the reverse when extracting live ammunition intact, and also fit a variety of slightly different bore diameters easily without jamming too hard in the tight ones.