6mm PPC --> 6.5 Grendel --> 6mm ARC - Why?

[BluntForceTrauma]

It's my understanding that Bill Alexander (primarily) designed the 6.5 Grendel as a whitetail hunting cartridge for the AR.

Yes, this. Bill will tell you he designed 6.5 Grendel as an AR15 deer hunting cartridge, but <wink> he had always hoped it would be adopted by the military in some way as a COTS (Commercial Off-The-Shelf) cartridge.

If the SEALs like the 6mmARC that they're playing with, Bill would kind of have been vindicated with the fat case, skinny bullet concept.

Actually, Bill saw some potential in 6mm and designed a "6.0 Wulfgar" based on the .222 Remington case shortened slightly and necked to 6mm. Never took it to market and, in hindsight, my opinion is that 6mmARC is a better way to do a 6mm in the AR15.