HaGaon HaRav Chaim Kanievsky says that Americans may carry guns into a synagogue on the Sabbath in order to defend themselves.
According to Yeshiva World News, Harav Chaim was asked about having armed guards in synagogues in light of the increase in anti-Semitic attacks. He responded by speaking against posting guards but made clear that worshipers could carry a firearm into the synagogue if it meant saving a life.