I was stumbling around the web when I ran into this surprise:
A Colt ACR prototype for sale. If I recall correctly this was designed to shoot duplex ammunition, 1 bullets in each case to attempt to gain a higher hit ratio. Attempting a mechanical solution to a training problem.
This machine gun is listed for at for $75,000 over at Machine Gun Central.
It has some interesting features. You can see the precursor to the SOPMOD stock. It appears to have an early flat top upper with a built in rear sight that flips up. It is my understanding that the Army’s Quick Kill training liked having a continuous line between the rear sight and the front, to aim similar to using a shotgun rib. I believe that is why the hand guard it shaped at it is.
It is an interesting bit of history.