Valkyrie Armament has developed a belt-fed system for AR-15 rifles. Any mil-spec AR-15 can be converted to make use of the system. The company sells AR-15 rifles already converted, although most customers prefer to supply a rifle for conversion.
The design is an improved version of the
Stoner 63 early AR-10 belt system. A M249 SAW ammunition box is held in place by an adapter that fits into the AR-15 magazine well. To switch feeding from a belt to a standard AR magazine, all that needs to be done is to open the rifle by the rear take down pin, push the mag release, pull the belt feed tray out of the top and close the rifle.
A similar concept to the Valkyrie is the Ares Shrike system. It is a belt-fed upper receiver for the AR-15 that, unlike the Valkyrie, uses its own proprietary gas piston and barrel attachment system.
UPDATE: The owner of Valkyrie Armament told me that the conversion price is $3,300 is you provide your own rifle.