Matt, I'd guess the original poster got his answer, yes, the 6mm Norma BR is a good 600 and 1000 yard "caliber" (actually "chambering"). Plus some others, in the Dasher & .300 WSM.
Which leaves us free to talk of other things. As soon as I hit send on the last post, I wanted to add that the BooBoo variants have a capacity of 88 grains water. The .300 WSM is what, 81? So the Boos are almost identical to the older .30-338 and the .308 Norma magnum, smaller than the .300 Win Mag & of course, the larger magnums.
One thing I discovered with the Boo is that while it would take Rel-22 or even Rel-25 and shoot some stunning groups, I got more consistent accuracy with 4831. Pursuing that, the .300 WSM tends to use 4350, right? What Bob Rosen uses in his .300 Ackley, but most others don't.
So while there is no proof, I think what's going on is that a balanced round produces more consistent accuracy, and records fall because when one of those is small, it's spectacular.
Well balanced means, with a .30, bullets between 190 and 210 grain weight. With the 6mms, it seems to be 95 to 110 grains weight. Same with powder, with the 30s, 4350 and 4831 seem to out-perform the others. I'm less sure of 6mms, though I had a 6 Ackley that won either group or score for 18 out of 20 relays over a two year span, shooting 4831. As a wild-ass guess, a bit smaller chambering using 4350 or even 4895 would have done better.
It's nice to have case capacity that forces you to powders that don't use tricks. But there is always Bob Rosen, using 4350 in his .300 Ackley. And from colum B, I have a .338/404 that will shoot with Rel-25. But you know what? It shoots better with 4831 and a lot of space in the case.
I've felt this way for several years now. Right or wrong, using that analysis, we're building a .338 based on the .375 Ruger case, picked because it's size is perfect for 4350 and 4831. The velocity will be whatever it will be, I'm not concerned. I also got interested enough to buy cases for a 7.5x55 Swiss for 600 yards, again because 4350 would be the perfect powder for it.
I don't believe in magic cases. People always mention the 6 PPC for short range. Then the .30 BR proved just as accurate, though not as easy to shoot. And yes, I'm talking group as well as score. Or Michael Turner's .30-30s, and smaller rounds based on shortening up the .30-30 case. It not magical, either.
The beauty of the .300 WSM is you just buy it and shoot it, using powders that don't have to use tricks to get the burn rate slow, where the pressure in the barrel is lower at the muzzle (i.e., when the bullet exists), etc. etc., and there are plenty of well-balanced bullets available to accommodate any barrel preferences.
I don't know the 6mms as well, though I'd think a 6x7 Lapua with a LR primer using 4350 might be just as good as the BR or Dasher.
FWIW