I have found that concentricity problems are more likely to be in the brass itself, not in the dies. If a case has .0 runout coming out of the sizer die, you cant fault the die. I tried many seater dies with equal luck, and when it was bad, it was always traced back to the brass being the culprit. When seating a bullet, it will always move to the weak side of the neck. Whether it is thicker on one side or just stiffer, it will always move to the weak side. Another cause is the case mouth being slightly out of square, tipping the bullet as it seats. It also helps to use a low angle chamfer on the mouth to reduce the starting pressure.