Before someone says that ignorance of the law is no excuse, let me say that I am well versed on that particular tenent of the law. However, in general, in my part of the country and I suspect in yours as well, it results in the defendant's being excused. In nearly all cases, either the prosecutor or the judge will take lack of intent to break the law, into account and will not go forward with the prosecution, if a reasonable average citizen could not reasonably have been expected to know that they were breaking the Law. In other cases, the jury usually will.
There are literally thousands (perhaps tens of thousands ??? who knows?) of AR15 type rifles with unsuspecting owners, that have M16 parts in them! Do you really think that the Gessstapo want all of those tax paying citizens who are guilty of no other crime and with friends and families who vote, prosecuted and put in jail?? Even in todays lunatic world, when something stranger than yesterdays event is sure to happen toda, I do not thing so.
Now a person who **knowingly** and intentionally violates the law by putting say an M16 carrier assembly in his (or her) AR15, might get prosecuted if they were charged with another coincidental charge and the part was discovered....which is another unlikely event unless the person was charged by the BATF for some other infraction. IOW, if you are a drug dealer, importer, or manufacturer perhaps you should know the different parts and check to be sure that none of them are in your AR.
If OTOH, you are a doctor, lawyer, Indian Chief, cook or bottle washer, who in their right mind is going to believe that you knew the difference?.....Even if the highly unlikely happens and an authority figure finds out?
My advice would never be to encourage breaking the law (which is probably a crime in itsself), commiting treason, becoming a tax protester or any other high profile person. These people are shot first! OTOH, I do not intend to become an armorer or weapons expert and go inspect all of the internal parts on my AR15s. I'm just too ignorant on the subject I guess. And that's what I would tell the judge......For a hell of a lot less than it would "cost to defend myself against the massive Federal Government" (more in the neighborhood of what it would cost to change out the parts that you mentioned).
Those who run around willy nilly shouting, "The sky is falling..The sky is falling" are not doing the rest of us a service......but then I don't suppose that is their intent in the first place. To the Will Nilly shouters, I would say, "Not likely".
Sometimes ignorance is a defense. Don't ruin it by helping publicize the details of the law and stampeding the novices.
PigPen