This is a traditional teaching of the Church: that you can defend yourself. This is morally allowed. Our father even had some (guns) for the duty to defend his children and his wife when they were attacked by other people who would want to kill them or harm them for their life or to endanger life.
And so in these extreme cases, defend only from killing. You can defend yourself, with this as an appropriate means. In this case, a gun or what you have, but only to defend; you're not doing other excessive things to defend yourself. So in this case, analogously, when the congregation is attacked by a terrorist and you have a weapon, you can defend yourself and the congregation from this terrorist.
Dear visitors of Gloria Dei and all who have already written or who will write a personal message to me. I am grateful for your attention, your kindness, for your spiritual support and especially for your sacrifices and prayers for me. God may reward you!