This is interesting, have you considered???
Guys. all this would be great but why not really take advantage of developments and go w/the 6.8 SPC.
From the information that I've read from Dr. Gary Roberts (DocGKR) the 6.8 SPC is a really devastating cartridge even in the shorter 10.5" barrel configurations, which one cannot say in regard to the 5.56 cartridge.
The 6.8 SPC would offer better incapacitation and barrier penetration over all existing 5.56 rounds.
Hell, this would be even better if you used the MGI multi caliber lower receiver. You could use their quick change barrel upper or dedicated uppers, and in any case you could use 5.56 NATO, 5.45, 7.62x39, 6.5 Grendel, .50 Beowulf, 9mm, .357 SIG, .40 S&W, 10mm, .45 ACP, and 6.8 SPC.
I think I just talked myself into another project !!!
"If war is ever lawful, then peace is sometimes sinful."
"Me? I think all of that is a load of gun school crap. I train and teach to shoot them to the ground. Beginning at the chest I shoot a burst to the chest and run the line up til I get to the face. Winning their hearts and minds....Suarez style."