None of us believe that the anti-gunners are just gonna say, "Oh, well Montana (and whatever other states decide to show the same resistance to fed control) said they aren't gonna go along with federal law so we'll just pack up and go home."
We all know that this isn't a panacea. But, look at Cali. L.A. allows illegal immigrants, in fact welcomes them and is a "sanctuary city" and the feds do nothing. In SF, you can poke smot all day long, and you'll never see a day of jail time as long as you don't deal the stuff. And the DEA doesn't breath down the necks of old hippie types there.
Montana figures that people in the state who decide to own a homemage cannon, or a silencer won't be attacked by the fedgov as long as they shoot on their land only, and don't make a big stink about what they are doing.
Is it ideal?
No.
But neither is the Cali situation ideal. (I would argue that while the state has no right to allow the illegals in, it's a Constitutional violation, whole part about invasion, they can do whatever they want about pot because the Constitution mentions it in no way, therefore it is a state/local government issue). Thing is this. How many states will follow Montana's lead. If a whole region in the country decides to do this, the fedgov might just be in a real rock and a hard place. Besides the left coast, no where else has the state government reversed pot laws on the federal level.
If say, Montana, Wyoming, Idaho, Alaska, Utah, Arizona, and Nevada all hopped on board with this stuff, the fedgov would find itself in a major pickle.
(This is part of why I really hate the Amendment that stopped state legislature voting of Senators, if the states had been electing people who believe what Montana's legislature believes, things would be much better in this country).