- makes it unlawful for any state or local officer or employee (such as police, prosecutors and judges) or any federal firearms dealer to attempt to enforce any federal law relating to personal firearms, accessories or ammunition owned or manufactured in the state and that remain in the state; and
- makes it a felony for any federal agent to attempt to enforce such federal law; and
- authorizes any person in violation of a such federal law to request the attorney general to defend him or her for such violation; and
- that any new federal law that restricts ownership of a semi-automatic firearm (or magazine of a firearm) or requires its registration, shall be unenforceable in the state of Missouri.
State nullification of federal law is the legal theory that individual US states have the right to invalidate any federal law that the state finds unconstitutional. In the early years of the republic, nullification was considered by many states, but the federal courts–not unexpectedly–have not upheld the doctrine.
Obviously, if it came to a showdown, the federal government might be able–to some extent–enforce its firearms laws in Missouri. What would really gut federal enforcement efforts, however, would be the lack of any assistance from state and local law enforcement. Having the Missouri Attorney General defend citizens prosecuted by the United States Attorney would also be interesting.