I think the PETN is there for the ball powder used in M80 ammunition. Back in the 90's it was known that m80 ball was experiencing ignition delays which could be problematic in the m134 minigun. Various primer/powder combinations were tried, and the recommendation was go to with the WC846FS (Flash Suppressed) powder as it showed better ignition with either the #43 or #34 primers.
I'm still learning about powder/primer combos, trying to figure out what works best. My experience with ball powders has been very good accuracy wise, so I looked a little deeper into where that "stick powders are more accurate" common wisdom came from, and that led me down the path of primers. Evidently the lots of match ammo loaded with 4895 were more accurate than the lots loaded with 846, and looking back with hindsight it makes sense that the powder/primer interface with 846 was a tad on the poor side.
I'm thinking that the whole "stick powders are more accurate" wisdom can be summed up in the story of the primers. Stick powders are generally easier to light, so you can have a smaller (hence more consistent) primer charge, which leads to less initial pressure variation (and more consistent timing). The powder isn't the culprit, the primer is the culprit as far as I can tell.
The problem is making a primer hot enough to light off ball powder consistently, that is itself consistent enough for accuracy. I think some of the new "green" primers will get there and we'll keep seeing the accuracy difference get slimmer and slimmer....
Machine guns are awesome until you have to carry one.