RE: The theory behind the 6.8mm Remington SPC
The 6.8 SPC has a .423 bolt face - or about 1/2 way between a .223 and a .308. So the AR-15/M-16 swap entails a new bolt as well as a new barrel. I have built four of these so far - three were AR-15's and one involved reworking a Ruger 77 in .223.
The whole issue, as I understand it, is that our military is currently using the 62 gr. NATO round in battle; and this round is not a very good stopper. Great tactical penetrator, though. If our troops were using Hornady TAP ammo (loaded with Hornady V-Max bullets), which is a very common round in civilianlaw enforcement these days, we wouldn't even be considering the 6.8 SPC. Even the cheap Federal American Eagle 55 gr. FMJ .223 is a pretty good fight stopper - and would, as I understand it, be Geneva Convention legal to use in warfare.
The rounds we carried when I was in the military 35 years ago were 55 gr. FMJ's coming out of a 1:14 twist barrel. Marginally stable, these bullets would start tumbling upon impact and create an impressive wound channel. If the first M-16's used in the Vietnam conflict utilized the current 62 gr. NATO round, I would expect our troops would have almost immediatley rejected it and gone back to the M-14.