This is IMO of course.... but the 556 case is probably about perfect for the 556 bullet but if you want a bigger bullet you need more powder behind it.. Now the dudes that developed the 6.8 took al that stuff into consideration and came up with what they think is the perfect combo and I am inclined to believe them... Now if I want to just go and blast a lot of rounds I use my 556 upper if i want to go and do work on stuff I am more inclined to grab my 6.8.
Molṑn Labé http://www.dtic.mil/ndia/2008Intl/Roberts.pdf
"I figure we are gunman because we are better then most, and it beats branding cows or digging copper" -Appaloosa
Arms discourage and keep the invader and plunderer in awe, and preserve order in the world as well as property... Horrid mischief would ensue were the law-abiding deprived of the use of them.-Thomas Paine
Handloader magazine #255 had a write up a few months back. It covered 6x45 in both a bolt gun and AR, with lots of loads.
Other than the cost of surplus 5.56 brass where's the cost saving? Custom barrel, semi custom dies it will cost more.
Now, if you just want to experiment that's understandable. We all have that bug :wink: