Saw this nice piece at a MG shoot. The owner told me it was all original Colt experimental belt fed. Which I would think is very rare. I personally have never seen one except in pictures. What I thought was strange was that the gun is obviously built on a Semi lower that was converted to full auto. The lower receiver is a Colt SP1, still marked FIRE instead of SEMI and has the large front pivot pin. It was quite impressive. He ran it a few times and I saw no malfunctions.

Note that the Russian steel case seen in the photos is from an AK. I saw him use the cheap Winchester white box stuff in this piece.

You can see in the picture above, the cuts required in the lower receiver. There is a big cut required on the right side and a not so big one on the left side. Also you can see the shape of the link "chute" of the ammo box. Pretty neat. All the links go in the ammo box on the right side and it is partitioned in the box to separate the live ammo from the spent links. Also, the whole belt feed mechanism pops out and you can use regular M16 magazines
Interesting. Regular bolt and carrier except carrier is slotted for the belt feed mechanism.
Note the ejected brass in the upper right hand corner of the picture. Unfortunately, I didn't bring a video camera with me.