WC846 had a hard time being lit properly by the #43 primer, there was an investigation (because delayed ignition would be a problem for the M134 minigun using M80 ball). Once the arsenal primer was upgraded to ensure no delays after 1986 I haven't heard of any gripes with accuracy until the change from WC750 powder to Reloader15 for M118LR.
It should be noted that the accuracy standards for match ammo were met using ball powder, which is from a rest. From a rest things like "ignition delay" don't show up on the target as readily as from a shooter trying to hold onto a target. So blaming the lack of accuracy solely on the propellent is a very "intuitive" answer, but it isn't the whole story.
Machine guns are awesome until you have to carry one.