Well; you say that guns are good because if a revolution is needed is better if the citizens have guns; the problem is that in revolutions the civilians who doesn't want it also have guns; so it gets really violent without affecting any "tyrant".
Of course it depends on the circumstances. Typically a tyrant maintains power by buying off a small portion of the total population. It can be as little as 10%. Somebody like Marcos in the Philippines comes to mind.