A separate question is why people need assault weapons in the first place. The answer given by gun rights advocates is that they are necessary to defend against tyranny.
There are a few things to say about this.
This is not supported by the 2nd Amendment.
While an individual right to gun ownership has been affirmed by the Heller decision, there is nothing in the Constitution to justify the “defense against tyranny” argument. The purpose of militias as set out in Article 1, section 8 is “to execute the laws of the Union, suppress insurrections, and repel invasions."
So the 2nd Amendment, created to ensure the maintenance of a well-regulated militia, was designed with suppressing insurrections in mind, not fermenting them.
Not that citizens have ever had much luck taking arms against their own government, no matter the number of weapons involved. I have more to say about this in a 2014 post comparing the importance of the 1st and 2nd amendments.
Many gun owners would not recognize tyranny if it bit them in the ass.
We have a president who calls the press “the enemy of the people,” goes out of his way to condemn peaceful protest (Kapernick et all) and equivocates in the face of Nazis and Klansmen, yet none of his supporters seem to recognize this as a slippery slope to any kind of tyranny. The reason for this is that tyrannical, illiberal regimes do not rise up by sending jackbooted thugs to VFW meetings and county fairs; they use patriotism, populism and demagoguery to exploit cultural capital and set groups against each other, making core supporters feel protected and free.
Most Germans in the Nazi era did not feel tyrannized; they stood by as others bore the brunt of oppression. Meanwhile the basic protections of civil society and a free press were pulled out from under them. Oh, and they had guns (Nazi gun laws deregulated the acquisition and transfer of rifles and shotguns). They had them, but had no desire to use them, because the Nazi party was widely popular.
What are you gonna do, really?
Let's say your worst fears come true. Red Dawn paratroopers. Blue-helmeted UN invaders. The Khmer Rouge (of course, the Khmer Rouge went after liberals, intellectuals, artists and urbanites, which ought to make you wonder why despots find these type of people—not NRA types—so threatening). In your apocalyptic scenario, did we lose the military, or are they on the side of the patriots? If the regime has military support, you’re toast. If not, then what's your Bushmaster for? Do the tyrants have air power? Can they simply obliterate you, the way Germany obliterated Warsaw? What’s your game plan, man?? Hide in the bunker until smoked out, or blown up? Is this how you preserve freedom?
Here's a better idea. Fight for transparency, equal protections, science-based policy, good government and constitutional rights (beyond weaponry), and against corruption, oligarchy, money in politics, vote suppression and illiberalism. Time and energy much better spent, and no chance of accidentally shooting yourself in the foot.