Properly expand your brass with the K&M Custom Sized Expanding Mandrels for the correct neck tension, before the seating process starts, and it also helps getting the brass case mouths perfectly cylindrical, on the inside, because that is where it matters, this is where your bullet makes contact with your brass, therefore you want to work from the inside to the outside when prepping case necks.
Commercial DIE manufacturers have no set standards for the expander ball/button in their DIES. Large variations from manufacturer to manufacturer and even within a given manufacturer makes it necessary to properly size and expand the inside diameter.
Expanding mandrels is the best and most accurate way to achieve correct final neck tension no matter if you are sizing full or neck only, bushing or non bushing, ensuring a more even and consistent seating process, reducing variances that directly influences both speed and groupings giving you more accurate data to record. This can also be used without turning brass for specific neckwall thickness and you will see vast improvements in consistency.
We find it best to size your neck 0.002” to 0.003” below your desired final bullet tension size if you are making use of bushing DIES, if not just remove the expander ball/button from your sizing DIE. The neck inside diameter needs to be slightly smaller so the expand mandrel can properly expand the brass, and be sure to always use a light lubricant.
Don't forget about purchasing a Press Adapter if you don't already have one which is needed for the expanding mandrel to fit into. Also the Window Riser is the most consistant way to go to complete the set.