I have an upper and when I bought it I was told it had M4 extensions , but it doesn't. So I have heard you can still have an rifle upper and an M4 barrel, but it isn't ideal. I do have the skill I believe to cut out the feed ramp extensions. But before I do that I want to know if anyone has run an AR with that combo (Rifle upper, M4 barrel) and if they had any problems.