A right to bear arms for the purpose of countering an autocratic government-gone-wild was acceptable back when the citizen soldier in aggregation with neighbors was effective enough to field a force of equal numbers with comparable armaments. That is no longer the case in this world. What preserves the union today is the institutions and the philosophy of the society. Public input into governance, separation of powers, independent judiciary, independent press, civilian control of the armed forces and a vigilant populous organized to resist any wayward move by the central authority by using those institutions.
The weekend warrior citizen soldier, no matter how many there are and how many bullets they could fit in the clip of their 9 mil and their automatic assault rifle, is not an effective counter to what would be fielded by an errant despotic regime.
The only defense we have in these modern times is us, paying attention, getting informed, getting involved, voting, being willing to go to court, supporting those private groups (on the left, on the right and everywhere in between) that watchdog and court challenge every move this government makes.