For ARs, I prefer Varget powder. It's very accurate and meters very nice in a powder measure. You can't over charge a case because the powder will spill out before you reach a max charge for a 55 gr bullet.
The most important issue with ARs is the tendency to get the chamber too hot from increased rate of fire. Varget is rated as an extreme powder, meaning it won't increase the burn rate much when it gets hot. Other powders do increase in burn rate dramatically as the chamber temperature climbs. This will increase chamber pressure and velocity. The result is a loss of accuracy and possible damage to your gun, plus hard extraction leading to malfunctions.
You want to avoid the long extruded IMR powders because they will foul your gas port. Besides, they are way too sensitive to temperature changes. Winchester ball powders like 748 work well provided you keep your cycle rate low. They too are temperature sensitive.
I use 26.0gr of Varget with 55gr Ballistic tips or V-Max. Accuracy is awesome and they function perfect.
