The fundamental tenet of the gun rights advocates is that, armed citizenry will take down tyrannical governments.
No, the fundamental tenet of gun rights advocates is that self-defense, which includes the right to own the tools of self-defense, is a basic human right. That fact remains true whether tyrants can survive armed populations or not.
Now it is true as a matter of history that one point the "Founding Fathers" considered was that a nation that relied on a militia (armed and trained body of citizens) for its defense, rather than a standing army, had a built-in defense again its government going tyrannical.
These 3D printed cheap plastic guns are going to flood Africa and other such places with very cheap guns.
Guns are already cheap and easy to make. That's the whole design philosophy of the AK-47 -- you can already get them for around $50 in some parts of Africa.