You use a lapping bar, either buy one or just use a 1" piece of good steel bar stock, and some lapping compuond and lap the bottoms of the mounted rings so that they align with each other. Missaligned rings will put your scope in a bind and can cause you scope to fail. It will also leave nasty ring marks,(dents) in your scope tube. It's not hard to do and usually only takes a few minuets. Well worth the trouble. KN