If you have to ask, the US Constitution is law. You might want to find a copy and actually read it. It authorizes the US Congress to create laws under it's authority. Congress writes the laws, and the President signs them. And if someone disagrees and believes the laws are unconstitutional, they can either write Congress and get them to repeal the law, or sue and have the Courts rule the law as unconstitutional. Finding a Constitutional Law class taught by lawyers who practiced that field might be a lot more enlightening for you than some random alt-right types from a conspiracy theory board.
If you honestly believe the NFA and other gun control acts are unconstitutional, what have you done about it? Seriously. Posting on a gunboard and making yourself sound tough isn't doing shit. Saying fuck it, I'm going to do 'civil disobiedince' isn't fixing the problem either. Also claiming you will violently resist the government isn't civil. Unless you also think the ANTIFA/BLM riots are civil too. You might want to read up on Henry David Thoreau, the guy who coined the phrase civil disobiedence. He was happy to go to jail for his principles. Anyone really wanting to go to prison as protest? Because that is what he, and people like Ghandi, MLK, Rosa Parks, and others who actually practice civil disobedience, have done for their causes.