Drooling over the Colt Military website, the Colt Automatic Rifle catches my eye...

In the overview, it says that it fires from the Open bolt position.

How would an open bolt AR work? i've seen the Armalite videos on the operation of the M16 so i kinda know how it works in closed bolt.

Nakey

I once shot an open bolt AR.  Long time ago.  IIRC, the bolt was notched out at the bottom, and the trigger retracted a piece (a "sear"??) that was where the hammer traditionally is.  It worked just like a MP40 or a "Greasegun".  The bolt keeps going back and forth until the trigger is released and it catches in the notch.  But I can't remember how the front of the bolt was configured.  What actually popped the cap...It could have been a sliding assembly within the bolt....but it's been too long to remember.