A refusal to enforce federal law is not what I call nullification. Making it illegal to enforce federal law within a state is what I call nullification - and it'll never fly. Like I said, Missouri has such a law but I have as yet to see anyone thrown in jail or even fined for enforcing federal law.
Refusal to enforce is the definition, but alright. Even so, it happened when the northern states refused to follow federal laws requiring slaves to be sent back to their owners, and it happened when states refused to take part in prohibition. It also happens every time a jury decides in favor of a defendant, when the juries refuse to recognize the charges as crimes (which happened in about 60% of all prohibition cases). Also, the verbage of both Kansas's and Missouri's pro-2A laws actually use the words null and void. To nullify is to make something 'null'...