Contrary to at least one report, the Montana bill was not a “nullification” bill. Nullification is an unconstitutional tactic where a state attempts to simply invalidate a federal law by enacting a law saying it cannot be enforced. The Montana bill, by contrast, merely refuses to permit state officials to enforce a federal law, while still allowing federal officials to do so. Denying state assistance to enforce federal gun laws is not unconstitutional, it’s just a terrible idea.
Several state’s are considering outright nullification of federal gun laws, however. A few of these bills have even passed state legislatures, and some would throw federal law enforcement officers in jail for enforcing federal law. These bills aren’t just terrible, they are also unconstitutional. The Constitution provides that duly enacted federal laws “shall be the supreme law of the land,” and thus valid federal gun laws trump state law.