Eugene Stoner, of AR-10 and M-16 fame, went on to develop a modular rifle in the 1960s. Carbine, rifle, machine gun, in various combinations for different missions. The system, when chambered in 5.56 NATO, is known as the the Stoner 63.
There are a handful of transferable examples out there. Here's one that will configure as a rifle, carbine, or Bren. Oh, and it's only $125,000. I'd say that makes it a grail gun. A few pallets of ammo and it's a day at the range!