From what I've read, the 606 was never technically 'fielded' but was tested under combat conditions. I would guess it would have shared the same fate as the 607, which wasn't fielded, but tested by SEALs. The 606 was basically a HB 604, and IIRC, the only real difference was it may have been an open-bolt design. It was heavier than an M16, but didn't give any appreciable firepower advantage, as opposed to the belt-fed version, which basically was up against the M60, and that was a no-go since at that time Army brass was happy with the pig. The 606 was part of the CAR-15 family - here's a pic. The 606 is on right but a belt-fed version. I was thinking it was the left hand rifle, but I believe that's a standard XM16E1. The Colt LMG and M231 were both open bolt, and neither had a FA, so i believe the 606 didn't either. I am wondering if the belt-fed version of the 606 had a different nomenclature.
IIRC ( I really need a copy of TBR) the 606 was fielded with regular mags for testing and it was like 'why?' which led to the belt-fed version, again the response was 'why?' A great concept that went nowhere. PS - I'd love a belt-fed retro M16!
Even a Ciener conversion. I wonder if you could make a SHRIKE retro?