I have a loading of 20.5 grains of AR Comp, under a Barnes 70 gr TSX, with a 2.250 overall length, basically max for a STANAG magazine. My notes showed it shooting 1.5 inch groups at 50 yards, from a 16 inch lightweight barrel, with a red dot sight.
I use either Reloader 15, or AR Comp for my heavy bullets, I did see an improvement in accuracy from my bull barrel when I switched to AR Comp with a 77grain Nosler.