You don't need to win a war to defeat a tyrannical government. All you have to do is to make it uncomfortable for them to proceed with their oppression. Private, legal ownership of firearms makes them "uncomfortable", which is a good thing. Government should fear the people in which they serve so they don't overstep their bounds.
Uh, here's the problem. Tyrannical governments usually have the enthusiastic support of their people.
Germany had plenty of guns under Hitler... Nobody used them to take out the Nazis... When the SS came for their Jewish neighbors, they all sat in their houses and hid.
What they did do with those guns is join the
Volksgrennadiers at the end of the war and fought against the allies to the last man and little boy. And after the war, a few of them became wehrwolves, until the allies confiscated all the guns.
So, um, no, I don't trust the wisdom of average people with guns all that much.