My suggestion is to buy a forster BR full length die(send it in to have the neck honed to your desired neck tension)a wilson hand die and an arbor press then you wont need to worry about concentricity.IMHO...and personal experience...the Hornady concentricity gauge will cause you more issues than you already have because as your chacing and adjusting run out your tweaking the necks and causing inconsistent neck tensions which cause fliers and higher extreme spreads especially if your adjusting up to .016 of run out. I have a hornady concentricity gauge that hasnt been used in about 4yrs...i talked to several old BR guys and most all said the same thing"buy better dies"! And one told me"you dont shoot BR dont worry about run out"he also told me to load 10 rounds exactly the same and adjust run out on 5 and shoot all 10 over a chrony....the 5 adjusted had a 35fps spread the 5 not adjusted had a 20fps spread.
Another suguestion is to do everything you can to eliminate cartage inconsistencies, do everything EXACTLY the same every time you load and most of all work on your trigger pull and position behind your riffle.