I suggest you read all you can on the 6 BR here:
6mmBR Cartridge Guide within AccurateShooter.com
At the bottom of the article they have frequently asked questions. One is titled: tight neck/ turn or no turn neck:
NOTE: they prefer LAPUA brass and suggest the reamer has dimensions that work best with that brass. If you are serious you will have your own reamer made. I think it is a very important aspect to creating an accurate rifle. It is well worth the $140. I use Pacific Tool and Gauge. After reading the articles you can call Dave Kiff and discuss the dimensions on YOUR reamer.
You won't get donuts if you don't turn the necks. Or if you turn the necks and do it properly you shouldn't have a donut either. The trick is to cut a portion of the shoulder when you turn the necks.
From the tight neck/no turn article in frequently asked questions:
"The collective wisdom is evolving on this issue. Originally, 6BR shooters emulated what works for the 6PPC–tight .262″ necks with a .009″ or so neck wall thickness. Now the trend is in the other direction. Many shooters have had great success with .268-.269″ necks that allow full neck uniforming with a cut up into the shoulder to block the formation of doughnuts. That said, we are now seeing very accurate rifles being built with .271-.272″ and even .274″ chambers that allow Lapua 6mm BR brass to be loaded and shot with no neck-turning. The results have been encouraging to say the least. Richard Schatz recently set a 1000-yard world record with a .271″-necked 6mm Dasher. And there is evidence that the longer VLD bullets work best with the greater neck tension allowed by unturned or minimally-turned brass. One well-known shooter says he has seen no loss of accuracy since changing to no-turn necks, and that’s how he will chamber all his future barrels. However, most top smiths still recommend turning necks, but not down to .262″. A .269″ chamber appears to be a good compromise, allowing a quick, easy one-pass turn without sacrificing beneficial neck tension. For a varmint or tactical rifle, the choice is clear, however–go with a no-turn neck so you can spend more time shooting and less time reloading. Both the Lapua and the Norma brass are good enough that you will give up very little accuracy."
There are many articles on the 6BR here, enough to keep you busy for hours.
As to the bushing dies. I prefer Redding S bushing die and a separate body "bump" die. The combined FL/bushing die doesn't allow the flexibility IMO.
I have used this concentricity gauge and really like it:
SINCLAIR CONCENTRICITY GAUGE | Sinclair Intl
Read all those articles and register on forums at that site. You will get a wealth of information from the best BR site on the web. Forums:
SHOOTERS' FORUM - Index
I am certain you will hear from others here. Good luck on your quest.