It’s very, very rare for mass shooters to be stopped by an armed civilian. There have been cases (such as when an Uber driver with a concealed carry permit potentially foiled a mass shooting in Chicago in 2015). However, in the event of a mass shooting, armed citizens could actually make the situation worse by confusing first responders and people around them about who the shooter is. In addition, gun owners are more likely to accidentally shoot a family member than they are an intruder.

Guns are not a defence against tyranny. The rule of law is the only defence against tyranny. The idea that armed people could overthrow a professional army is laughable; it might have worked in the 18th Century, but the capability gap between civilians with small arms and modern armies with tanks and stealth bombers is now a yawning gulf.

Guns are a necessary tool in a violent world. They provide protection from muggings, home invasions, terrorist incidents, and mass shootings. Furthermore, an armed citizenry is (as a last resort) a guarantee against tyranny. Should it ever come to insurrection, then the people would have the means to rise in armed revolt against the authorities and overthrow them. Even if it never comes to that, the threat of armed rebellion will help preserve government from tyranny.