Actually, I think you might be mistaken.
From what I have seen, the AR-15 and M-16 I believe have a great deal in common, structurally and functionally.
The difference is how the “lower” is machined or drilled out. (the “lower” is what the Feds regard as “the gun”) and as such, it is the one that is stamped with identifiers.
Usually.
From what I understand, it is technically quite easy to change an AR-15 semiautomatic to a full automatic if you are willing to break the law and have the resources to make or buy the small part(s) needed to accomplish the conversion.
I am sure there are those who know more about this kind of thing than I do.
I have often wondered if their whole obsession with the AR-15 has as much to do with that fact (that they could be converted) as the emotionally scary (to them) black color of the firearm.
I guess I should have made clear that they have little in common where it counts...semi automatic fire vs fully automatic. One is for war,the other is for self protection and other lawful activities.
Yes it would easy to convert one to full auto, but most of them would melt the barrel and injur the shooter.
Have to admit I play with rubber bands sometimes....