Not even one. He wouldn't get anywhere near a door before he was laying in a pool of his own blood on the ground.
And that is why he would drive past Sidwell to the nearest “gun free zone” at the public school / shopping mall / movie theater / park. Unless he is after a specific person or organization, by increasing the odds just slightly he will die before he gets the chance to take someone with him, he would find another target. What is so hard for liberals to understand about that?