Not enough info to diagnose the problem. Did the round jam up as you tried to chamber it or after it had fired? Also, whose M855 were you shooting? Federal? Olympic? Radway Green? What does thea headstamp say?
On the rifle, what kind of chamber does it have? Who manufactured it? What markings are on the barrel?
Off the top of my head, it sounds like you fired 5.56mm ammo in a .223 match chamber and the brass stuck to the wall from overpressure.