I just got a look at the FightLite Industries MCR Belt Fed AR-15 Upper Receiver.  Unlike other models, this bad boy only requires you to replace the bolt hold open (part provided) and you don’t have to cut up your lower to make it run.

The MCR allows you to convert your AR-15 from a standard, magazine fed rifle, into a multi-purpose belt-fed, that can also feed from a magazine. Simply attach the upper as you would any other, link up your ammunition and go to the range. This unique upper receiver includes a Picatinny rail mounted to the top cover, as well as Picatinny handguards. 16.25” quick change barrel features a chrome lined bore & chamber. 1-7 twist stabilizes standard, military ammunition, and ½-28 muzzle accepts standard muzzle devices. Receiver is machined from 7075-T6 billet & features a Type III matte black anodized finish. 6-lug rotating bolt is more robust than a standard AR-15 bolt, and is designed for sustained fire roles. Feed tray is steel. Bolt carrier is semi-auto. Upper utilizes M249 SAW links. Upper weighs 7.45 lbs & has an OAL of 32.25”.

MCR Complete Upper Assemblies