For accuracy, never use extender buttons....ever..they will create donuts in your brass....when you fire a case, it will straighten to conform to your chamber. Use a full length neck bushing sizing die that allows you to bump shoulder of case back about .002 and choose a neck diameter bushing and expander mandrel that proves most accurate for you load. After sizing you case(use lube for smooth resizing), then get a couple extended mandrel’s and use them to control your neck tension. 21st century makes these in 1/2 thousand sizes. This is important..your extender mandrel is what controls your neck tension, not your neck bushing die.
My routine- I pull fired straight from box and anneal with my AMP. I then full length size, bumping shoulders and sizing neck( I remove extender button from die and use a neck sizing bushing that is .002 under diameter of the expended mandrel I’m using). Now I use the extender mandrel to open up every case neck. Once this is done, I tumble the case in a walnut media....bare in mind that I’ve used plenty of imperial wax on cases while sizing. When you tumble the brass in the media, the wax off the case will collect on the media lightly-this will actually lightly lube/clean the inside the neck cases while clean the outside of cases. Under sizing the necks and then using the extender mandrel, along with annealing and tumbling your brass which cleans and lubes inside the necks, makes every cases neck tension as close to one another as possible. You control neck tension with neck sizing bushing and extender mandrel..and every gun is different on what it likes for neck tension...let the gun tell you what it likes...I hardly ever run over .0015 tension on any target rifle that I own..
My no turn neck 284 for example. A loaded round neck measures .315 with a Berger 184. You should check this anytime you change bullets types or weights. I use a .319/.320 chamber reamer when chambering which gives me .004-.005 neck clearance total. I use a .311 neck bushing in my full length sizing die, then follow with .313 size extender mandrel. My case necks now measure .314 before seating bullets. This gives me exactly .001 neck tension on ever single case assuming every case is annealed the same and every cases neck thickness measures the same all the way around. Turning necks to control this is another subject.
I shoot very little benchrest...most long range F-Class. I average 3000-5000 rounds a year. I hope I made some sense above..