New Frontier Armory (NFA) is shipping their C-5 AR receivers. Not just another Colt clone, these guns run MP5 magazines. The big advantage to this is that the MP5 magazines are great in carbines and rifles. The Colt magazines were hard to load and prone to failures.
I have been working with Pistol Caliber Carbines (PCC) since the Colt SMG. I have always liked being able to shoot economically and train on pistol rated steel targets. PCCs have gotten steadily better over the years and I think the C-5 may represent the pinnacle of their evolution.