The forum is on the money! All you need is a *complete 6.8 SPC Upper and a Magazine and you have a full fledged 6.8 SPC Cal. Rifle.
Also for example if you wanted a 458 SOCOM? All you would need is a complete 458 SOCOM Upper. Even takes the standard 5.56/223 Magazines. That is what makes the AR-15 Platform such a great unit. This also applies to other calibers that are available with only the same type installation.