A supersonic load for the Sierra 220 grain MKHP is 15.6 grains of AA1680 with a 7 1/2 Remington primer loaded in A Remington 300BLK case to a over all length of 2.190 . A 16 1/2 inch barrel S&W AR 15 was used to record A average velocity of 1520fps. A Oehler 35P chronograph was used to record these velocities . The the air tempature was 70 degrees at the time they were recorded . This load was safe in the rifle used for these test there were no FTF or FTE the bolt locked open on the last shot. As always use at your own risk . Start at 10% under listed maximum load and work up.