It doesn't have to, all it has to do is make the cost of oppression too high to sustain, and when there are millions of firearms in the hands of civilians, that cost can get very high, very quickly. Why do you think the Russian military was chased out of Afghanistan by a bunch of nomads with AK-47's? It wasn't because their military was inferior, it was because the Afghans kept chipping away at them, taking out a few soldiers here and there, knocking down a chopper by getting above it as it flew through a valley and dropping stones on it, then melting back into the countryside. Eventually, Russia decided it wasn't worth the cost and left. The same would happen with a few million rednecks armed to the teeth and hiding in the swamps. Then, to make it even more difficult, if an American democrat president really tried to use the military against US civilians, there would be mass defections and some of those advanced arms would end up in the hands of said rednecks.
That's just a numbers game. How successful would Ruby Ridge have been if there were several dozen rednecks hiding in the trees, shooting back at the agents?