The mishna in Shabbos mamkes clear that weapons of war are inherently not Shabbos’dik. And they probably had in mind someone wanting to wear fancy parade armor. Furthermore, if a terrorist can easily identify who has a weapon, they know who to target first. If it is necessary for civilians in their community to carry weapons on Shabbos, it is best to conceal them.
And I seriously question how many frum Jews have the sort of training to be able, in a crowded shul, to hit a moving target that is firing back at you, but that’s a different matter.