6.8 Great 300YD and under gun much more capable then the 5.56 or .223, plus many types of factory loaded ammo to choose from.
6.5 Great 600YD gun, can be reloaded at normal reloading costs after the brass is purchased.
Shooting at 400yd plus is not a plinker, so not much ammo is used.
I am looking for a 400YD plus gun, being able to swap uppers is a plus for me.
With a longer barrel for distance and much heavier it's not a replacement for the 5.56 but a new tool for longer ranges. The 6.8 is a replacement for the 5.56 300yds and under as I see it.