Shalom Harav, It seems to be clear from Rav Goren in Mashiv Milchama Vol. 2 pg. 56 that he only believes that a solider or other member of the security forces can carry a gun on Shabbat. This also seems to be his opinion in Trumat Hagoren Vol 1 pg. 153. If that is the case as a private citizen how can I use the explanation of Rav Goren to carry my firearm on Shabbat?
Rav Goren holds that guns are muktza if one is not using it for reasons of security; say if your wife or children were to want to move them or somebody wants to pose with it on Shabbat. But it’s different when you are asking about carrying a gun to contribute to the call of the authorities that as many civilians as possible should walk around armed. Halachically, there’s no difference if you’re wearing a uniform or not (like a plain-clothed policeman), and if your role is security, it’s allowed! How much more so according to the opinion of R. Shlomo Zalman Auerbach (Shmirat Shabbat KeHilchita 20, note 28), who accurately opines that the real and more common purpose of weapons is not to fire them but to deter the potential terrorist or villain. Accordingly, guns have the status of kli shem’lachto l’heter (!) v’lissur, that are not only used for actions prohibited on Shabbat, but also for needs that don’t (!) involve m’lacha and don’t violate the Shabbat, and are accordingly not muktza and are allowed. Thank God that we live in a time where Jews can and do protect themselves, and don’t just “fight with their passports”!