Just the bottom the top is thinner and more flexible. DavidI understand it is an argument to lap and not lap. I understand precision machined rings shouldn't need it but if it were a one piece ring like the nightforce direct mount then i would see no need to lap. I plan on doing it for this rifle as it is my most expensive to date. If the lapping goes bad and wont hold the scope then i will take off the rings and get a set of Spuhr mount and 1913 base and bed the base. But for lapping do you do just the bottom ring or put the top of it on and lap both top and bottom?