It used to be, if you had an AR and any M16 parts in it (unless they were modified to Colt SP1 configuration), you were considered to be in violation of the NFA.
Your safest bet if youre not sure, is to skip the internet expert opinions, and call your local office of the ATF and ask. Dont call DC, start local. If they dont know the answer, which I'm pretty sure they will, they will refer you. The local people were always helpful for me. They just seemed to get grumpier and less knowing as you got closer to the DC area code.