I’ve owned this exact upper for 4 1/2 or so years now and I’ve put probably close to 10,000 rounds through it and I haven’t had a single malfunction with 5.56 ammo of all kinds... it hates .223 but that’s probably more to do with my buffer choice. This gun runs M855 all day long and has amazing accuracy... I put a 4x12x45mm scope on mine and i haven’t out shot it yet. I would take this upper into conflict.