It’s not actually about winning against the military. The civics justification for having guns is to make harassment campaigns more accessible when necessary. (Any sustained resistance resistance campaign would have to have outside supply lines.) No modern rebel group has taken on an established military on equal footing. The goal is to make oppressing the population extremely annoying, not to actually be in control yourself. In order to actually run a government you need a different set of skills than to run a resistance campaign, but a resistance campaign might become necessary until we can restore the government to a just one.

There’s other justifications for individual ownership of firearms, but that’s the one most similar to what you’re thinking of.