I know this has been covered and rejected, but I would love to have a 6.8 spc necked to 6mm. Silverstate armory is making lots of quality brass that is reasonably priced, and I really dont think that the 6.8 will go away anytime soon. There are simply too many ar's out there, and if you look over on ar15.com, you will see that it has taken on a nice following. I know I love mine. easy to load for.
6mm-6.8spc has already had some decent development, and would allow the use readily available inexpensive magazines. the only problem I see is the need to wade through the ar15.com clutter to find the in formation.
That being said, I like your hornet idea too. Seems less blah than 6-223. ymmv
Bullets have well documented performance specifications.
Min Velocity info is available, and describes the performance floor of the bullet... DON'T allow yourself to reference F.P.E. as an accurate descriptor of killing power.. You are smarter than a copy editor, and you understand that you can determine maximum (reliable) effective range by comparing your drop chart to published bullet data....... And, dog-on-it, people like you!