I would polish the stem so that the edge of the opening does not make contact with the bullet, so that you have curve on curve contact, and if you are dealing with new brass use an expanding die and mandrel no the necks to bring seating force into a more reasonable range. For work on seater stems, given that I do not have a lathe, I chuck them in my 3/8" corded Makita drill, spin them up to 1,000 rpm, and use a light touch with some fine wet and dry abrasive paper. By turning the work, and given that this drill's chuck seems to be pretty well centered, the results have been quite good.