The DRD Tactical U556 Take-Down Upper Kit will be shipping soon from Brownells.com. The U556 system allows the barrel/rail assembly to be quickly detached from the rifles upper receiver. It also allows quick barrel changes.
The DRD Tactical U556 Kit includes a forged upper receiver and Quick Detach rail assembly. A barrel, of any AR-15 compatible caliber, and other upper parts (bolt, stock etc.) must be supplied by user.
The MSRP is $630, which is less than I expected. I know that the manufacturer has already shipped a batch of them to Brownells, so I expect they will be on sale very soon.