We had these only we called them "Mini-16s" or 177s. MACV-SOG had some...so did some rotor jockeys and tankers. The official name was Colt Commander XM-177. This was the prototype of the M4. (Incidentally, I carried for a while an XM-174, a mag-fed select fire 40mm grenade launcher, which, I believe, was a prototype for the Mark 19.)