CMMG makes good stuff. High quality ARs. My CMMG M4 LE 5.56 shoots around MOA with several loads, with no mechanical issues. I can’t tell you about that particular model, though.
6mm ARC is the 6.5G necked down. It will give you similar to .243 ballistics out of the AR platform. It’s a really good little round, but not sure about how available factory ammo is for it since it is no longer the newest kid on the block. If you handload then you are GTG