Wouldn't the first act of an evil government be to outlaw civilian armaments and confiscate these guns? At that point, you'd be forced to either go to war(which you agree you would lose), or lose your weaponry. Perhaps the proliferation of guns would help the now criminal resistance acquire an arsenal, but I'm skeptical. European history is rife with examples of armed resistance forming from an unarmed populace.

I wouldn't call my ancestors 'helpless' considering their multiple armed rebellions and constant resistance after being conquered, or examples I'm less acutely aware of like the French or Polish resistance during WW2. It's also worth noting that while this resistance was very much detrimental to the hostile government, it was never the primary factor of its downfall.

If it comes to that, whether there's a legal right to have said guns is pretty much moot, because the people you're using those guns to resist aren't going to respect that right. If you're prepared to fight your government, you don't care whether it's allowing you to have the means to fight it.

Therefore the right to bear arms as a defense against tyranny is mostly empty words. The way to prevent tyranny is robust public institutions and a democratic culture. Weakly democratic states fall to autocrats all the time. There is no culture of democracy so the number of power holders a would-be autocrat has to cow is few. Democracy, real democracy, involves orders of magnitude more people, it's that much more difficult to overthrow.