An AR will work better and longer if you stick to a powder at the faster end of the suitable burn rates. The reason is the remaining pressure when the bullet uncovers the gas port. You want that to happen when the pressure curve is as flat as possible, so the pressure is as consistent as possible at that instant. If the port is uncovered while the pressure curve is still steeply descending, the slightest variation in timing (bullet velocity) at that instant will give widely differing impulses to the gas piston and concomitant differing vibration patterns in the rifle. At extremes, the bolt can be slammed back hard enough to rip the rim off the still-expanded and firmly stuck cartridge case!
If I had an AR shooting normal weight bullets (50-55), I'd feed it powder in the 4198, Benchmark, H322, RL-10X, 2015 area.