It was designed to add more stopping power to the M16. The guidelines where for it to be a simple conversion in the M16, Ie barrel swap. That being said it had to use the same bolt. the 5.56 Nato has a .378" case head, so the new round must as well. So what you have is a 223 necked up to accept a 270 cal bullet. I think that a 6.5 (.264) would have been better suited. It may be an inferior round in you eyes, and if we're talking hunting I would pick a lot of cartridges ahead of it as well. In a firefight if I had to choose between the 5.56 nato and the 6.8 SPC I would take the 6.8 hands down. But, I would probably be searching for a M14 shooting 7.62 Nato, also known as the 308, which is a far better round.