Armednfree,
If you don't like the look of that ring mark, just imagine how upset you would be if you could see the bullet leaving the muzzle with all those gross rifling marks swaged into it! The ring has no effect on exterior ballistics for the same reason the rifling marks don't change the BC of the bullet; they are covered up by the boundary layer of air that travels with the bullet in flight. That layer is due to the fact air molecules are attracted to and actually stick to the bullet surface and try, less hard, to stick to the air passing by, and the two influences collude to attract a following of molecules a few thousandths of an inch thick that form the boundary layer. In a bullet, that layer is turbulent, and if you ever get to look up close at the shadowgraphs of flying test projectiles in the late Robert L. McCoy's book, Modern Exterior Ballistics, you will find you can see it quite clearly and even make out the turbulence in it. Thin, but there.
It is not true that only the base matters, but it matters more to group size because small errors in the base interact with muzzle blast to introduce drift away from the ideal trajectory path, thereby opening groups up. It takes a much bigger nose distortion to do that because the influence on the nose is from the headwind and not the brief tailwind of muzzle blast. The headwind is continuous, so whichever way it pushes the bullet is reversed every half rotation of the bullet, undoing much of the previous half turn's error. Thus, as Harold Vaughn showed, He could cut a 45° slant on the nose of a bullet and only open the group up by about a quarter of what a mere 2° slant on bullet base caused. Harry Pope famously remarked that "the base steers the bullet". Over a century later, that is still true. But it must be added that the nose is largely responsible for determining ballistic coefficient, with a perfectly pointed base having around a third of the effect on it that the same shape nose does (you can test this by firing bullets backward). Each end plays its role.
So, the ring is a purely cosmetic issue for unfired ammunition. In order to accept long bullet noses, the recess machined into the ramming end of the seating stem of your die has a steeper angle than the line tangent to your bullet's ogive where the two meet. If the mouth of the recess is left sharp, that edge both shaves and impresses the ring into the bullet.
As already mentioned, if you use one bullet all the time, you can sacrifice a couple of them as laps to alter the seating ram mouth with fine lapping compounds and polishes. It doesn't take much. A small fraction of an inch of matching profile will spread the load out enough to mostly eliminate the ring. If you want to try that and haven't ever done lapping before, I recommend drilling a few quarter-inch holes in some scrap steel and practicing. You want to work the lap back and forth and not turn it in just one direction if you plan to avoid making circumferential scratches on it. You want to turn the work a bit every few lapping strokes to average out the pressure angle of the lap, or else have the work rotating slowly as you do the lapping in order to get the most perfectly even job you can.
The simpler thing to try first is to chuck the seating stem in a drill and turn it slowly while you dull and polish the edge of the mouth of the ram. A Dremel tool and Craytex tips will do a fine job of that. Just progress through the grades of the tips from coarse to fine until that mouth edge is rounded and smooth. When you are done, the ram still contacts the bullet over a small area, but it is no longer sharp so the ring indentation has a gradual side slope and fresh subsurface copper is no longer being exposed. This leaves it much less pronounced.
