In the AR 15 platform, about the best most powerful cartridge you can use is the 6.8 SPC or the 6.5 Grendel. Both have almost double the energy of a 223.
I chose the 6.8 because of the slight increase in energy for hunting, brass availability and mag interchangeability. When I finish testing different bullets in the 6.8 I will post the results. the rifle has already shot 1/4 MOA with 110 grain bullets at well over 2700 ft/sec and testing is just getting started.
The larger 6.5s require a AR 10 lower (Like the 260 rem and the Creedmoore.
Ether choice is a good one and you will be happy with the performance
J E CUSTOM