Shooting a 5 inch group at 1000 yards is a worthy achievement. The Williamsport crowd shoots 10 shot groups at their matches.
I should mention that I expect the gun to be capable of holding .5 MOA, not me.

With small calibers like this, the wind is brutal at long range! I have shot some pretty impressive 3 shot groups under ideal conditions but I stop before I ruin them with any more shots.

We have built a few 6 brx on the ar15 platform. They are fun guns and make a hell of a coyote rifle.
What bolt/barrel extension did you guys use and how are they holding up? That seems to be the real limiting factor with .473 bolt face cartridges in an AR-15.
I am kind of in the same boat here. Looking to build a 6mm AR also to replace my 243WSSM...This would work great except I cant get the billets seated without damaging the ogive due to the excessively thick necks. I need help here so any suggestions would be great. already have a sinclair collet die, but even with it, there is ~4 thou of neck tension after running them thru.
I have been asking around, and from what others say ($0.02) JP barrels are so-so. the JP bolt is good, but Kreiger or Bartlein, or Benchmark would be my barrels of choice.
You could either inside neck ream or outside neck turn the brass for the WSSM. Outside turning is easier. I would think that 2 thousandths neck tension would be pretty good. For barrels, check out Blackhole Weaponry. They're priced right and I have one on my CZ 527 264 LBC that shoots great!
Andrew