The Supreme Court's ruling in Heller, which first recognized an individual right to own guns, said that the Second Amendment didn't apply to "dangerous and unusual" weapons, but didn't clarify what they meant by that. Most lower courts since Heller have said the exception applies to fully-automatic weapons, but SCOTUS has yet to revisit the issue.
Dangerous and unusual is impossible as the AR 15 can be fired by and has to be by everyone who is in the military. The AR 15 cannot be unusual if almost everyone who ever served since about 1966 has fired them.
A footnote in Heller noted that if M16s could be banned, then the 2nd A would be meaningless - ergo, any such ban is flatly unconstitutional.
Being a footnote it carries no legal weight, but does constitute a “don’t go there” warning.