It look to me like some of the plating is being stripped away as the bullet leaves the throat of the chamber, stack up a few rounds worth and that ring is the result. My guess is not enough flare before seating the bullet and/or too much crimp (with plated bullets it should be just enough to remove the flare ~0.378").
Jacketed bullets are less sensitive to those things (and more expensive), but you should be able to shoot plated bullets (I've shot over 10K).