Earlier today I was thumbing through the latest Book of the AR-15 at the newstand and saw a story on the MGI Hydra which I'd never heard. That piqued my interest so just now I went to their website and watched this amazing video.
Their system allows quick barrel changes using standard AR barrels and that's not all, their lower receiver has a removable magwell so you can turn your 5.56 into 7.62x39, 5.45x39 and many other popular calibers. Of course some calibers require a bolt change too but not all. The price really isn't bad, about $1300 for the basic 5.56.
Anyway, it's well worth thumbing through that magazine and/or watching this 4 minute video: