An AR-15 SBR in 6.8 for hunting (Deer, Coyote and maybe Hogs), have a lower and looking to put parts together for the upper. Looks to me like this is THE place for this caliber in an AR platform.
Almost forgot, this build will be suppressed as well.
Not a SBR guy but I would look hard at ARP15 barrel. Look at their website. I think he recommends Mega uppers but it could be Aero. Anyway I am sure you will hear for more knowledgeable folks. Welcome to the forum.