Colt also made a 9mm smg for the DOE that fired from an open bolt, but that bolt would not function in a 5.56mm carbine/rifle.
There are real experts over in ARFCOM/Retro who can tell you more [
http://www.ar15.com/forums/f_3/123_.html] but basically your 'local gun expert' is blowing smoke up your butt...