Any person untrained in using firearms in a combat situation (i.e. more than shooting at a target, or even an animal) is not likely to be of much use fighting a terrorist, and probably will injure himself (e.g. “shoot himself in the foot”) or others (i.e. “friendly fire”).
One might also argue that the derech of Yaakov is not to show off and display a gun on Shabbos (or any other time) but to keep it out of sight. Boasting publicly that one is ready to kill terrorists serves only to “dare” them. If one is authorized by the Rav (or President, or Gabbai, etc.) to have a gun in shul, it should be known only to them, and to anyone else with a gun – it shouldn’t be displayed. The Kol Yaakov in this case should not be discussing firearms (unless a terrorist shows up, in which case he may shoot him and go back to davening).