I finally had a chance to experiment a bit with the problem and the RCBS seating die I referred to in the original post. I neck-turned a few cases using my old Marquart neck turning tool which pretty much eliminates run-out on the neck O.D. and creates consistent neck thickness. I seated a few bullets with lock ring locked in place and got the same .003-004" run-out. Then I seated a few bullets but with the lock ring completely loose which resulted in just about zero additional run-out. As the bullet enters the case mouth the die flexes a bit but at least with those bullets, a Sierra boat-tail match, it made a huge difference. Perhaps it will be different by using conventional hunting bullets, but will report later. Thanks all who have shared.
