Colt ACR, built to fire duplex rounds. It is said it had 40% less recoil than a M16A2. Note the stock which was a predecessor to the SOPMOD stock.
Much to my dismay, some years ago, I saw a near complete set of Colt ACR parts for sale individually. A bunch of different people bought individual parts. I think it is a real shame one person didn’t buy it all to build a semi auto one.