I just neck down 6mm Creedmoor brass with mine. I use Starline SRP brass since Lapua didn't make it when I built my rifle. I use mine for hunting with a suppressor, so my barrel is only 20" long. The barrel is from X-Caliber, 1-7 twist 5R rifling, and have been very happy with it.
I tried using RL 26 powder, but didn't have good results with a shorter barrel than most are using, but H4350 & RL 17 work great in it. I mainly shoot 75gr ELD-M's out of it, but also shoot 80 & 88 grain bullets.
The best group (5 shots) I've got with it is .179", but generally it'll shoot in the .3" range.
It's a fun gun to shoot!