It's a modification of the old .30 Remington, same family as the .25 Remington and .32 Remington, as used in the Browning-Remington autoloading Model 8/81 rifles, and the pump 14/141 rifles.
They in turn were rimless ballistic clones of the .30 WCF and .32 Winchester Special, and loading data from the latter can be used for the former.
the 6.8 SPC still has a pronounced shoulder, so headspacing and extraction is really no problem.
To fit into an AR-15/M16 length magwell, those .277 bullets have to be short, so you won't be able to run the high BC numbers like the Berger .277 VLD without intruding into powder capacity.
I wanted to build a 6.5 SPC wildcat, and run .264" high-BC bullets, but again the length and powder capacity limitations keep me in the lightweight bullet territory.