On the 6.8mm topic.
There's also the factor that the 5.56mm has bee found a bit lackluster when it comes to stopping power. Of course everyone would like every weapon to have the stopping power of a .50cal!
If the USMCs current move to replace their tube artillery with various types of MLRS systems is fully adopted it will have "interesting" repercussions on picklists, costs, and artillery overload.