You have to consider the fundamental differences between a nation having two armies invade it simultaneously, one from the west (Germany) and one from the east (Soviets) and these armies both being prepared to do incredibly brutal things to achieve their ends vs. what people are talking about.
When people talk about private gun ownership being a bulwark against tyranny in the USA, they aren't talking about simply some sort of direct contest of who has the superior firepower between citizenry and US armed forces, we're all very aware of the answer to that question.
There are other factors, like hesitation and uncertainty on the part of soldiers asked to attack their own people on their own soil, and the insurgency dynamic. The need for the powers that be to maintain the will to do what they're doing as it gets bloodier and bloodier, and if such a government at that point cared about any segment of the population's support for their actions (if they were only targeting certain portions of the population) that support drying up is another consideration.
So again, it's not just red coats and blue coats line up on opposite sides of the field and may the side with better guns win.
And also, even if we were to concede that such a scenario (US government trying to kill a crap ton of it's own people) was super far-fetched, I still believe that it is positive for politicians who are forming policies related to our civil liberties and our freedoms to have such a scenario as a possibility in the back of their mind, even if they aren't aware it's there. The desire to avoid such a bloody affair can reign in abuse of power. Disarming the populace emboldens abuse of power by those who wield it, in many if not all cases.
