183 / Can citizens who own guns ever put them to good use?

I would like to ask anyone in the American pro-gun lobby, who often justify the right to gun ownership with the argument that "individuals need guns to protect themselves and the public from gun carrying criminals," whether there is any evidence at all, that random shooters, like the Tucson murderer, were prevented from carrying out a mass killing by an upright, armed citizen who used their own gun to good effect. SMH letters