4001, 4002 (or C variant now) and 6001, 6002 are going to be the most popular due to that's what most people "need" for general purpose bolt guns. The other models are more 'specialized' for more specific uses. The next most popular models will be the 20 MOA variant of the above mentioned models. Most users have a 20 MOA rail and don't need the 20 MOA variant unless their rail is 0 MOA.