Can't think of a good way to explain both other than they are both situational in PvP. PvE wise like Explosive rounds since (in my own mind they deal solar dmg) and the fact enemies tend to group up (Hive mainly) it can help with quick clearing of areas. APR are more for those narrow areas like the rooms enemies come out of when your defending the ghost or running in the abyss with lots of thrall chasing you. but even then Explosive rounds do just as good.