Probably a little outside the box you guys want to play in, but isn't the highest BC bullet for an AR-15 among these?
Berger 130 VLD .552 BC
Hornady 123 AMax .510
Lapua 123 Scenar .547
Sierra 123 MK .610
All 6.5 projectiles, available in a 6.5 Grendel.................
Cheers