I have a 6.8 upper made by the company formerly known as Ko-Tonics, and now very nearly formerly known as Cardinal Armory. Fortunatly, there’s a new kid in town making 6.8 SPC gear known as Bison Armory. Steve from the Firearm Blog shows off some of their stuff. I enjoy shooting mine when I can get it out, and I’m still using the brass I collected from GBR II back in 2007. Fits on any standard AR lower.