My son has a Bushmaster AR it has a 24 inch barrel with a 1/9 twist with a free floated barrel. I am having trouble getting heaver bullets to group well. Using the 69 grain Sierra Matchkings it is shooting almost a 5 inch group at 100 yards, this is using Varget. It shoots about the same group using the 55 grain Noslers, and again with Varget. It will shoot a much tighter group using Benchmark and the 55ers. I still think that it should shoot much better. Under an inch at a 100 easily. I have the same rifle except my rifle has a 1/8 twist and a 20 inch barrel. My rifle shoots 1/4 inch at a 100 using just about any load with Varget and it doesnt shoot the Benchmark very well. My question is: Is Varget too slow burning for that longer barrel? And any suggestions on a faster burning powder to improve on the group size?