I use an 18", a friend uses a 16" that was shortened from a 20". A 55 ballistic tip with 25.5 grains of H335 should shoot well.
I forgot about an upper that I loaned a friend, that has a 7" twist White Oak barrel, and the above load is what shoots well through that barrel. And, my wife's AR has another 7" twist White Oak barrel, and that shoots under 2" at 200 yards with a 1-4X Meopta on it with some old Black Hills 52 grain match loads.